After a lot of thought I revised the whole idea and fixed some potential problems. This mod is intended to be a companion mod to NoCubes.
I need a mod that adds blocks that are identical to every block already in the game. So there would be 2 of every block. The modded blocks would have a slightly different name to there vanilla counterparts, but for now lets call them "modded blocks". All the block drops from "smoothed blocks" would be "modded blocks". But all the block drops from the non smoothed blocks would be "vanilla blocks"
How this works with NoCubes-The idea is that if you break a smoothed block like dirt, you would get the block version from this mod in the form of a "modded block". But if you want the smoothed dirt you would need a simple enchantment kind of like silk touch which would give you the "vanilla block". If you were to break a block that is not smoothed like wood, you would get the "vanilla" wood block. But if you were to break it with the enchantment you would get smoothed wood which would be the "modded block".
This would make it so that all blocks the player breaks will be "blocks" so they can build without problems. However I found you can build very interesting shapes and structures, especially nice pillars with smoothed blocks. I think this would really help open up the creative building and help structures mix more with NoCubes. Who knows what other creative uses this may have, but I think this would be a nice addon to NoCubes.
Notes- Still need a name for the enchantment and what to call the modded blocks. Maybe "Smoothed & Cubed" would be a good mod name?