I think that I would check over the fuel system... I had a '50 Chev that acted similarly. I eventually found that the suction line from the tank to the pump was nearly full of rust flakes. The fuel pump could pull a bit a gas through while cranking, but went to instant vapor lock when the engine started. I temporarily ran a hose from the tank to the pump bypassing the hard line for testing purposes when I found that I couldn't blow out or ream out the line. This engine would start and idle for a few minutes, but couldn't accept any load or raised RPM due to fuel starvation.
