I'm assuming a Rochester carb is being used. Correct carb to spacer gasket? Please do not assume anything here...look at the vacuum passages, compare to your gasket and understand you may have to cut a notch or punch a hole in the gasket you're using to get vacuum running as designed. If it runs for 20 minutes and dies, I'd wonder if the gas cap isn't vented or if there is some obstruction in the fuel line (or the connection inside the tank) where crud is hampering the flow of fuel. A float adj issue wouldn't have it running that long. Remove gas cap and see what happens. As for buying a "rebuilt" Rochester from any auto parts store, good luck. There is almost a perfect likelihood anything you get will be at best a hodge-podge of assorted but not correct nor matching parts.
