The power of @DeleGate!

After.java

package tterrag.customthings.common.block;

import java.util.ArrayList;
import net.minecraft.block.Block;
import net.minecraft.client.renderer.texture.IIconRegister;
import net.minecraft.creativetab.CreativeTabs;
import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.ItemStack;
import net.minecraft.util.IIcon;
import net.minecraft.world.IBlockAccess;
import net.minecraft.world.World;
import tterrag.customthings.common.config.json.BlockType;
import tterrag.customthings.common.config.json.BlockType.BlockData;
import tterrag.customthings.common.config.json.BlockType.MaterialSound;

public class BlockCustom
  extends Block
  implements IBlockCustom
{
  public final BlockType[] types = new BlockType[16];
  private final BlockProxy<BlockCustom> proxy;
  private final BlockType.BlockData data;
  
  public ArrayList<ItemStack> getDrops(World arg0, int arg1, int arg2, int arg3, int arg4, int arg5)
  {
    return this.proxy.getDrops(arg0, arg1, arg2, arg3, arg4, arg5);
  }
  
  public boolean isToolEffective(String arg0, int arg1)
  {
    return this.proxy.isToolEffective(arg0, arg1);
  }
  
  public boolean isOpaqueCube()
  {
    return this.proxy.isOpaqueCube();
  }
  
  public IIcon getIcon(IBlockAccess arg0, int arg1, int arg2, int arg3, int arg4)
  {
    return this.proxy.getIcon(arg0, arg1, arg2, arg3, arg4);
  }
  
  public IIcon getIcon(int arg0, int arg1)
  {
    return this.proxy.getIcon(arg0, arg1);
  }
  
  public String getHarvestTool(int arg0)
  {
    return this.proxy.getHarvestTool(arg0);
  }
  
  public boolean canHarvestBlock(EntityPlayer arg0, int arg1)
  {
    return this.proxy.canHarvestBlock(arg0, arg1);
  }
  
  public int damageDropped(int arg0)
  {
    return this.proxy.damageDropped(arg0);
  }
  
  public float getBlockHardness(World arg0, int arg1, int arg2, int arg3)
  {
    return this.proxy.getBlockHardness(arg0, arg1, arg2, arg3);
  }
  
  public void registerBlockIcons(IIconRegister arg0)
  {
    this.proxy.registerBlockIcons(arg0);
  }
  
  public int getExpDrop(IBlockAccess arg0, int arg1, int arg2)
  {
    return this.proxy.getExpDrop(arg0, arg1, arg2);
  }
  
  public float getExplosionResistance(Entity arg0, World arg1, int arg2, int arg3, int arg4, double arg5, double arg6, double arg7)
  {
    return this.proxy.getExplosionResistance(arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7);
  }
  
  public int getHarvestLevel(int arg0)
  {
    return this.proxy.getHarvestLevel(arg0);
  }
  
  public BlockCustom(BlockType.BlockData data)
  {
    super(data.getType().material);
    setStepSound(data.getType().sound);
    this.data = data;
    this.proxy = new BlockProxy(this);
    setHardness(0.3F);
    setResistance(0.5F);
    setCreativeTab(CreativeTabs.tabBlock);
  }
  
  public void setType(BlockType type, int meta)
  {
    this.types[(meta % this.types.length)] = type;
  }
  
  public BlockType getType(int meta)
  {
    return this.types[(meta % this.types.length)];
  }
  
  public BlockType getType(ItemStack stack)
  {
    return this.types[(stack.getItemDamage() % this.types.length)];
  }
  
  public BlockType[] getTypes()
  {
    return this.types;
  }
  
  public BlockType.BlockData getData()
  {
    return this.data;
  }
}

Before.java

package tterrag.customthings.common.block;

import lombok.experimental.Delegate;
import net.minecraft.block.Block;
import net.minecraft.creativetab.CreativeTabs;
import net.minecraft.item.ItemStack;
import tterrag.customthings.common.config.json.BlockType;
import tterrag.customthings.common.config.json.BlockType.BlockData;

public class BlockCustom extends Block implements IBlockCustom
{
    public final BlockType[] types = new BlockType[16];

    @Delegate
    private final BlockProxy<BlockCustom> proxy;

    private final BlockData data;

    public BlockCustom(BlockData data)
    {
        super(data.getType().material);
        setStepSound(data.getType().sound);
        this.data = data;
        this.proxy = new BlockProxy<BlockCustom>(this);
        setHardness(0.3f);
        setResistance(0.5f);
        setCreativeTab(CreativeTabs.tabBlock);
    }

    @Override
    public void setType(BlockType type, int meta)
    {
        types[meta % types.length] = type;
    }

    @Override
    public BlockType getType(int meta)
    {
        return types[meta % types.length];
    }

    @Override
    public BlockType getType(ItemStack stack)
    {
        return types[stack.getItemDamage() % types.length];
    }

    @Override
    public BlockType[] getTypes()
    {
        return types;
    }

    @Override
    public BlockData getData()
    {
        return data;
    }
}

BlockProxy.java

package tterrag.customthings.common.block;

import java.util.ArrayList;
import java.util.Random;

import lombok.RequiredArgsConstructor;
import net.minecraft.block.Block;
import net.minecraft.client.renderer.texture.IIconRegister;
import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.ItemStack;
import net.minecraft.util.IIcon;
import net.minecraft.world.IBlockAccess;
import net.minecraft.world.World;
import tterrag.customthings.CustomThings;
import tterrag.customthings.common.config.json.BlockType;
import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;

@RequiredArgsConstructor
public class BlockProxy<T extends Block & IBlockCustom>
{
    private final T block;
    private final Random rand = new Random();
    
    @SideOnly(Side.CLIENT)
    private IIcon[][] icons;
    
    @SideOnly(Side.CLIENT)
    public void registerBlockIcons(IIconRegister register)
    {
        icons = new IIcon[16][6];
        for (int i = 0; i < block.getTypes().length; i++)
        {
            BlockType type = block.getTypes()[i];
            if (type != null)
            {
                if (type.textureMap == null)
                {
                    icons[i][0] = register.registerIcon(CustomThings.MODID.toLowerCase() + ":" + type.name);
                }
                else
                {
                    for (int j = 0; j < type.textureMap.length; j++)
                    {
                        String tex = type.textureMap[j];
                        icons[i][j] = register.registerIcon(CustomThings.MODID.toLowerCase() + ":" + tex);
                    }
                }
            }
        }
    }

    public IIcon getIcon(int side, int meta)
    {
        return block.getTypes()[meta].textureMap == null ? icons[meta][0] : icons[meta][side];
    }

    public IIcon getIcon(IBlockAccess world, int x, int y, int z, int side)
    {
        return getIcon(side, world.getBlockMetadata(x, y, z));
    }
    
    public boolean isOpaqueCube()
    {
        return block.getData().isOpaque();
    }

    public boolean isToolEffective(String tool, int metadata)
    {
        BlockType type = block.getTypes()[metadata];
        return type.toolType.isEmpty() ? block.isToolEffective(tool, metadata) : tool.equals(type.toolType);
    }

    public String getHarvestTool(int metadata)
    {
        BlockType type = block.getTypes()[metadata];
        return type.toolType.isEmpty() ? block.getHarvestTool(metadata) : type.toolType;
    }

    public boolean canHarvestBlock(EntityPlayer player, int meta)
    {
        BlockType type = block.getType(meta);
        ItemStack held = player.getHeldItem();
        int harvestLevel = getHarvestLevel(meta);
        if (type.toolType.isEmpty() || held == null)
        {
            return block.canHarvestBlock(player, meta);
        }
        return held.getItem().getHarvestLevel(held, getHarvestTool(meta)) >= harvestLevel &&
               held.getItem().getToolClasses(held).contains(type.toolType);
    }
    
    public int getHarvestLevel(int metadata)
    {
        return block.getType(metadata).harvestLevel;
    }

    public float getBlockHardness(World world, int x, int y, int z)
    {
        BlockType type = getType(world, x, y, z);
        return type == null ? block.getBlockHardness(world, x, y, z) : type.hardness;
    }

    public float getExplosionResistance(Entity par1Entity, World world, int x, int y, int z, double explosionX, double explosionY, double explosionZ)
    {
        BlockType type = getType(world, x, y, z);
        return type == null ? block.getExplosionResistance(par1Entity) : type.resistance;
    }

    public ArrayList<ItemStack> getDrops(World world, int x, int y, int z, int metadata, int fortune)
    {
        BlockType type = block.getType(metadata);
        return type == null || type.drops.length == 0 ? block.getDrops(world, x, y, z, metadata, fortune) : type.getStackDrops();
    }

    public int damageDropped(int metadata)
    {
        return metadata;
    }

    public int getExpDrop(IBlockAccess world, int metadata, int fortune)
    {
        BlockType type = block.getTypes()[metadata];
        return type == null ? 0 : rand.nextInt(type.maxXp - type.minXp + 1) + type.minXp;
    }

    private BlockType getType(World world, int x, int y, int z)
    {
        int meta = world.getBlockMetadata(x, y, z);
        return block.getType(meta);
    }
}