So, I'm updating the mod I've been working on to 1.8, but the RenderBlocks class that I used to render... blocks... into a custom GUI no longer exists. I switched to using RenderItem renderItemModel(ItemStack). It renders items fine, but it only renders blocks registered within my mod.

 

 

 

They all run off the same renderItemModel, leading me to believe there is another function I can use for blocks. But, I have yet to find it... Any ideas? :L

 

GL11.glPushMatrix();
        {
            GL11.glTranslatef(x, y, 0F);
            GL11.glScalef(20F * scale, 20F * scale, 1F);
            GL11.glTranslatef(.55F, .7F, 1F);
            GL11.glRotatef(180F, 1F, 0F, 0F);
            
            if(spin)
                if(itemStack.getItem() instanceof ItemBlock) {
                    GL11.glRotatef(20F, 1F, 0F, 0F);
                    GL11.glRotatef(Minecraft.getMinecraft().theWorld.getTotalWorldTime() % 360, 0F, 1F, 0F);
                }else
                    GL11.glTranslatef(0F, (float)Math.sin(Minecraft.getMinecraft().theWorld.getTotalWorldTime() / 20F) * .1F, 0F);
                
                renderItem.renderItemModel(itemStack);
        }
        GL11.glPopMatrix();

Take a look at GuiContainer and look at the drawItemStack method.

 

It should have you covered.