My .02

Vapor lock: he sees it in the fuel line filter. He's insulated the lines and it got better.

Might need to tackle under hood temps.

Wrap the exhaust, see what can be done to lower engine operating temp.

Pull the air cleaner and look for fuel boiling out the carb when engine is shut down hot.
If so, then you have heat soak on carb as well.

Put a pressure guage on fuel line, and see what is really going on, don't guess.
Running a regulator and return line can heat the fuel in the tank, and potentially be counter productive.

Relocating the solenoid is a stop gap measure. The better bet is to wrap the exhaust and install a heat shield.