350 is perfect...check with state emissions requirements, you might not be allowed to swap out a bigger motor? you might get stuck with another 305/350
anything bigger than a 305/350 and your going to have MAJOR problems with suspension and clearance problems
(actually a Big Block chevy will bolt right in, without any mods except exhaust and mabye some Minor hood clearence issues..
and the stock suspension will hold a BBC with an aluminum intake and water pump with No problems)
although the stock trans and rearend will be weak points ...