Here in Calif. you can do special construction, basically you say your built your car from various parts from scratch. This is one way. But the vehicle is denoted as SPCS or some similar code they will use for your make. This is best way for someone who just wants to register something least amount of hassle, and cares only about driving it and getting it legal for the most part.
If its a collectible, then this route will just devalue the car most likely. Might depend how much the buyer cares though.
If car is really old, I say at least 25 years or more. Go to DMV say you owned it originally, been in storage, you lost all your documents to a tragic fire and you need to re-register the car.
You probably just need to do a statement of fact, vehicle verification (possible CHP verification) pay tags, done deal.
Keep in mind every state DMV has different rules and laws. I would go to DMV in your state and ask the manager some questions.