Randy, you might want to check with ATFE or get a copy of the Federal regs. They're free from ATFE for the asking. Besides the length problem there is an obscure reg about making a rifle from a handgun. Don't know if that is still in effect but it came up when the ATF tried to ban the T/C Contender carbine. The courts rulled it ok to put the carbine barrel on a Contender pistol along with the stock but not with the short pistol barrel. If I remember correctly this is the only case to which this applies without going class 3.
Check it out, Dude, the penalty is NOT worth it.
Pecos