Yes I believe they'd be smog exempt, anything 1975 or under IIRC.
The later months of "1974.5 260z" as they're sometimes referred to are based off the 280z's somewhat beefier frame. So technically yes, maybe a little stronger, dunno about "better" per se. Alot of guys like the originality of the first model Z car, and like to do 240z v8 swaps for the prestige. Something about Z cars deviating from the original race inspired ideals as they were redesigned through the years. I'm no expert, that's just the feel I get from what i've read, hope it helps.