There are a few options you arent exploring here. 1) shaved heads. Shave your 350 heads down, Bolt them on the 305 and you will re-gain your compression while having better flowing valves. 2) serious rebuild. Rebuild the 350 and apply a stroker kit during the process. 3) exhaust. From what ive read from your posts, you dont have a high flowing exhaust. A must for a High Performance Small block. Finally 4) a cam change. My guess would be your running a factory 400-something lift cam. Or maybe even high .300s A little lift can go a long way