To the thread author's question: Yes you definitely can register it as a pistol with a 16" barrel, but
No, you can not put a sub-16" barrel or a barrel that will make it less than 26" in overall length on it later because it has a stock. If it has a stock and it has a barrel less than 16", it is a short barreled rifle by federal law. If it's less than 26" in overall length, and it has a stock, it is an AOW (Any Other Weapon) according to federal law, which would need to be registered on a Form 1.
If you really, really wanted to... you could make it an AOW, with a barrel longer than 16", but an overall length of less than 26", you would have to do that on a Form 1. I'm pretty sure you
can do that in Michigan, though, you definitely can
not make it a SBR, because that's specifically against Michigan law.
To GunnFixr: It can be registered as a pistol in Michigan because Michigan
law is retarded. In Michigan, a "pistol" is defined as anything shorter than 30", regardless of whether or not it has a stock. If it's under 30", regardless of whether or not it has a stock, it needs to be registered as a pistol.
*Editted to insert "law"