Your choice really all comes down to personal preference. For the V8 swap, some people like the 240z (low weight) and some (me included) prefer the 280z (once the awful "park bench" bumpers are gone ).
One thing that you really should consider is the fact that CA has horrible smog laws. I am not sure about the particulars, but I believe that you can only swap an engine that was offered in that model line (i.e. the 280zx engine into a 280z). Hopefully somebody who has already dealt with the smog ordeal can provide you with some information.
For ease of a swap, I would recommend getting a pre-1976 car.