[pat in Santa Cruz]

I think your problem was the cap. Watch out for repro gas caps too. There is a hole in the center that has to align with a hole in the inner cap. The center can rotate and shut off the vent hole. The one you have pictured is the style that can do that. Snyders makes a good one (so they say) that is a copy of the Eaton. I prefer an original. A show quality one is pricey, but a driver quality one can be found at a reasonable price.

Some of the gaskets today can also block the vents in an original cap. There should be an airspace between the outer edge of the gasket and the inner edge of the cap. The most recent gasket I got from a quality vendor was too large and the car ran well at 35mph, but failed within a mile at 55. When I unscrewed the cap, there was the tell tale sucking sound from the vacuum in the tank. I had to trim the edge off by about 1/8th inch around the circumference.

I drove my sedan up the Columbia River in a heat wave with a stock carburetor when it was so hot the fuel was boiling in the bowl when I stopped. It made it hard to start, but once started and cool fuel ran down the line, there was no trouble. The bubbles in a gravity system with a straight line to a vented tank do not get trapped and cause a blockage at least with an indented firewall type fuel shut off. It could be the vapor lock is more of a problem with the firewall mounted fuel sediment bowl.