Your fuel has a lower boiling point due to the alcohol content in it which makes it way easier for vapor lock and fuel boiling over in the carb. I had this on my 65. Whenever I got back from even a 20 minute drive, I could hear fuel boiling in the tank. My 3" TTI exhaust pipes were 1-2" away which did contribute to the issue but the fuel also burned hotter causing more engine heat. Fun stuff. Did you take the filler cap off and get a whoosh of air? May need to look into a way to make the venting better so the tank pressure can drop.