340

Sounds like the valve seats are beat out of it being it's an original 340 with std. valve seats. They weren't hardened and unleaded gas kills them. That causes the valves to not seal up. Pull the heads and have a look at the valves.

Also in the future if you have a tech question put it in the right forum. This is the local happenings forum and don't draw much attention to tech questions. You should have put this in the small block tech forum. Alot more guys would have seen it there.