Why won't my truck idle when hot?

I have a 1972 GMC truck with a problem. It runs fine when the engine is cold, but when the engine warms up the engine starts to surge when I stop. It runs fine as long as I'm on the gas, but when I let off the throttle, the rpms go really low and it tries to die, then it picks back up to a very fast idle, then back down again, etc. It does this until the engine finally dies. I was told that I have a vacuum leak, but I can't find one. I drove it until it got hot and started acting up, then I unhooked and blocked off the power brakes, modulator valve, and vacuum advance. No help. The engine is the stock 307 with a Holley alum. Intake and 600 cfm vacuum secondary carb. I was told maybe the intake was cracked so I changed to a new polished intake and tried a new quick fuel carb and a used edelbrock carb. No help. The truck had a 3 speed trans when I bought it and I changed it to a turbo 350 with a B&M 2000 stall convertor that I bought from a friend. I thought maybe the convertor was bad, so I bought a new 1800-2000 stall TCI one. Again, no help.
The engine has 71000 original miles so I thought maybe the cam had a bad lobe. I pulled it out and the cam and lifters look great. While I had it out, I installed a new timing chain. When I had the engine out to install the trans and a new chrome steering column, I took a look at the rod and main bearings (they looked fine), and installed a new std. Volume melling oil pump. I don't know if it had this problem before I bought it because it didn't have any brakes so I hauled it home and never tried to drive it until after I installed the new trans and new power brakes. It also has a HEI distributor with a flame thrower coil, cap and module. I don't see anything wrong with them and I just installed new AC plugs ans new plug wires. NO HELP AGAIN ! A friend says it may be the exhaust doesn't have enough back pressure. It has stock manifolds and the pipes are stepped up to 3" into dual flowmaster 40 mufflers then 3" tail pipes out the sides in front of the rear tires. I don't like this set up but it was like this when I bought it. I've done everything that I know to do. Any help would be great.