You're dealing with vaporlock in the heat. It was not an uncommon issue before fuel injection, especially on hotter days. Yes, ethanol gas could be contributing to it, because it has a lower boiling point than non-ethanol, it's not likely to be your entire issue. As others suggested, doing the fuel line mod is probably a good idea. I did it when I rebuilt my slant and swapped carbs, and had significantly less heat transfer into the fuel line during a drive.
Getting rid of heat is a big issue, your issue is at idle, and the radiator fan doesn't spin as fast at idle, meaning less air moving across it, and less cooling. That contributes to vaporlock at idle, as the heat of every component in the engine bay will steadily rise the longer you idle.
If you can get no ethanol gas, you might as well. It has a higher energy density (potentially more mpg), and doesn't have the tendency to build up water if it sits for a bit.
Edit: Ethanol has only 2/3 of the energy of gasoline, which translates to some fuel efficiency loss.
