Not really that I know of, there's a general CIII but that's not applicable here. I'm pretty sure others here in the BIY forum can steer you in the right direction for other questions.
As for the pistol build if you want to err on the conservative side, that's certainly understandable. When you've got the receiver how you want it you can have it engraved using NFA guidelines (should be .003 deep, at least 1/16 tall) include the manufacturer (your name) city/state it was made, serial number, and clearly state that it's a pistol. You can get creative or it can be something as simple as:
John Doe Anytown USA
UZAK Pistol 9mm/.45
s/n 1269
But you're right, having it clearly be a pistol could make a lot less hassle of you have an encounter with a LEO who isn't sure about the laws, even if he just held you up for an hour while things checked out, it'd be easily avoided if marked Pistol.