Tough call. A '75 350 is probably a low-compression motor, but for a truck application that might not be so bad. Is this thing gonna be a daily driver, or more for show, or what? If I was gonna drive it much, I'd put the 350 in. OTOH, if you were to fix up the 283 with correct period valve cover, air cleaner, etc than would look neat.
FWIW, if I recall correctly, the early (circa 57, 58, etc) blocks could be bored to 4.00, giving an original 302. Short stroke, with good cylinder heads it can be a real screamer. I built one once, years ago....
Good luck!
George
