The biggest problem would be the bore/stroke ratio. 307s and 305s were slugs for a reason--the bore was way too small for the stroke. The 302 screamed because it had a large bore--4"--and a short stroke--3". The 307 with it's 3.875" bore and 3.25" stroke crank created a lot of internal friction and the 305 was even worse. While this was adequate for a smog motor that never saw more than 4000 rpm as a rule, it's a disaster for performance. And as Mark said, the small bore will shroud the valves as well. Pretty much all a 307 block is good for is a boat anchor. Good luck!