Easiest by far would be an oldsmobile 350. same size and rough weight as the 307, all the hookups and brackets are the same. exhaust is the same. water pump, everything. It's dead nuts easy to build a 300hp olds 350. The "good" non windowed blocks are from 68-74ish. Good olds small block heads are #3, 4, 5, 6, 7, and 7A. Not 7A, those are crappy 307 heads, which you probably have. Look for the tiny A. 350 olds .030 over with decent hyper pistons, 9.5:1 (pump gas friendly, depending on where you are, if you have 93/94 at the pumps you can run 10:1 or higher with a good tune), a good cam, head work, and headers should make the mid 300s easily.
What transmission do you have? Does it have overdrive? Is it P R N (D) D 2 1 or P R N D 2 1?