It would be advantageous if you had a title for the body you are using. Cops don't see the frame when they are pulling you over for fictitious tags. Could you beat it in court? Maybe, but why have the hassle? Better to have your vehicle registered as what it actually looks like...
i would talk to your local dmv i think then can registar a title for the frame. if the vechicle goes through inspection you'll need some sort of proof that you legally own it