Well, perhaps you have changed the value in the wrong asm file.
Open the the asm of Davros' P-switch and change the value 0E there, where I stated before, to 04. That's the pow music number of the smb3 music patch. The fix patch only change the music of the original sprite.
Don't forget to reinsert the custom sprite.
You insert custom blocks in most cases as tiles, so they will always reappear, just like the standard coins. These only vanish, if you insert them as objects.
Well, there are now two ways to solve your problem:
1. Make a custom object with object tool from the patches section and let it use the level's inventory index (and your custom block, of course). I did not use object tool before, so I can't give you more hints about this way.
2. Change your custom block into a sprite and take a deeper look at this: