As an update:
The '91 comes from the year the Fiberfab body was constructed. My car was built on a '70 chassis.
I got it registered as a 2005 SPCNS (specialty constructed vehicle) in CA. The CHP restated the original VIN from the donor bug that had been junked, and I have a little metal stamped plate in the door jamb to prove it.
Because it's got a '70 motor, I took it to to the CA smog referee for an inspection, thinking it would be smog exempt. Well, sort of:
What? 1970 Beetles didn't have dual carbs and electronic distributors? Uh oh. So even though it doesn't need to be smogged, it can't have all the newer gizmos and geegaws. I'm not about to put on a single carb and a vacuum-advance distributor, sooooo...
I have temporary tags through the end of November. Car will sit in December. Then I'll be in line at the DMV on January 1st for a certificate of sequence.
I've been to the DMV 4 or 5 times, to the CHP, and to the Smog Referee. Even though I get different information from each of the agencies (and each trip) about what the title will eventually look like, so far the process has been relatively painless, rather efficient and most of the parties have been quite helpful.
I was worried that I'd have a difficult (or impossible) time getting the car properly titled and tagged, but it's been easier than I thought, even though obtaining a certificate of sequence was my "worst case" scenario...