Theoretically you can legally swap an LS1 into a 240SX. Problem is, if you have to modify or fab things up like headers, its not a legal swap. If stock everything fits into the 240 though, I'd recommend starting with a 96+ S14 for ease of adapting OBDII systems rather than trying to convert an OBDI car to OBDII.
And no on the SR, it's actually an illegal engine *everywhere* in the US, as it does not meet EPA standards. Thing is, you can get away with it in places that are not as strict as California, but it is still illegal to run technically.
And yeah I've considered trying to find a place to register my cars out of state as well, just to make life easier. However California has rules that if you have a residence in California and have your car here for more than 2 weeks it must be registered with the state of California. Retarded in just about every aspect, but the state is greedy, what do you expect. Still, they would have a hard time proving that you didn't "return" home if you left town for a day or so.
Oh new idea, group buy an apartment in Reno (or maybe Vegas! lol) for California residents! Or somewhere in AZ, or Oregon, all 3 have looser laws than we do (hell if you live outside the major population areas in Oregon, you don't even have to smog your car!)