I've not priced the combo out so I can't answer your Q.
I would go all the way to 347 cause why leave power on the table.
As for hp gain ...... if we are talking about mild street combos like some of the kind you find on this forum then the 302 ci motor in those combos will be in the 300 to 325rwhp range. Its not unreasonable to expect the 347 to put down 350 to 375rwhp if you keep things to an apples to apples compairson.