I want to let you guys know how it turned out. I went for a test drive with the old, original gas cap on the tank and that wasn't THE problem. Still stalling out when under full demand. It was a cool day and there wasn't any percolation in the carb so I could rule that out as well. I swapped out another known good running Rochester carb for the one on the 57 and that made no difference. I pulled the sending unit out of the tank and the filter was 80-85% blocked with rust. I could barely see any light shining through when I put a LED light right next to it. The entire inside of the tank that I could see is coated with rust. So, in the end, fuel percolation, altitude, and winter fuel blend were all blind alleys and the simplest answer was the right one. Thanks everyone for all your help!

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