318 Retards Under Load?

I don't think this is really your problem, but if the car bottomed out or something it may have a restricted exhaust. I say this only because it reminds me a lot of how cars act with plugged converters. Air filters can plug and do this too. I think you have a fuel problem though. Take the line off the carb and extend it enough to be away from the engine. Put the line end in a gallon jug, and let it idle till the bowl runs dry. If you don't get a half gallon or more you may have a fuel delivery problem.