Truck Forums - Timing advance?

Alright I have a 64 chevy with a 80 chev 350 engine. The transmission is a saginaw four out of a monza. The problem I'm having is when i shift from 3rd to 4th gear the rpm range drops from about 3500rpms to 2000rpms. When the rpms get that low and I'm trying to get on it, it pings like crazy. Accasionally it gets slight pinging in 3rd gear at low rpms but 2nd and 1st gear have no pinging what so ever. I have tried retarding the timing but it starts firing back through the carb in low gears and atidle before the high gear pinging goes away. The only thing i can think of is the vacum advance. I have no idea how to adjust it. Or if you even can. Any advice would much appreciated. Thanks, Kirt.