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Noah 930 has summed it up perfectly. The only thing I would add is that some regions of California do not require the bi-annual smog checks once the car has had the initial check/pass when it was purchased. This does give one the option of buying a post 75 car, pass smog upon registration, then make mods without having to worry about smog checks. Two problems, of course. One, you will have to either live or have an address in one of the exempt regions, which tend to be rural and far from metropolitan centers. Two, when the car is sold, it will still have to pass smog so all changes will need to be reversed.

Best bet, IMO, given the regs and your past experience with Porsches, is to go with a 75 or earlier car.