KW...you are THE MAN in the repro body industry.
What would you tell a customer headed home with a new frame and body about proper registration? I know it varies a lot by state...
I know lots of '32's out there, both on original and on repro frames, have highly illegitimate paperwork...old title purchased online, original Ford numbers punched into a new frame, aftermarket little tags with numbers punched in...all likely to trigger something between difficulty and felony down the road if someone with a good manual inspects the heap. What is the proper way?
Number 1. Check with your individual state for rules and procedures. Contact both the DMV and research the relevant statute and administrative rules (easily accessable on the web for the most part) as the DMV may not be familiar with uncommon stuff like titleing a hobbiest constructed vehicle.
Number 2. Keep all your reciepts, especially those for major components such as the motor, frame, and body. If you do your own build take photos. One of the key questions the DMV will have is do you truley own the stuff.
Many states are "very easy" and allow a car to be titled as to what it looks like either outright "1932 Ford Roadster", or branded "1932 Ford Roadster Replica" or "1932 Ford Street Rod". Generally states which generate income from title fees on a flat rate or a vehicle value are easiest. States like California which charge a very high fees for registration for new cars and low fees for older cars are often "hard" as their income is based on the year shown on the title.
Most states consider any part of the vehicle to be replaceable. In those states you can replace the motor, then the frame, then the body on an original titled vehicle and be legitimate. Rules for applying numbers to frames and bodies vary from state. Some prohibit the owner from doing it, some require the owner to do it.
For untitled original cars there are also, again varying from state to state, provision for issuing "state" serial numbers, bonded titles (allowing a "true" owner to claim the vehicle with a certain number of years), salvage titles, sherrif's titles, landholders titles (a landowner makes an affidavit that the vehicle has been abandoned on their property for many (like 30 in Oregon). Again, research both the statue (law) and administrative rules (DMV handbook of how to implement the law).