Electric fuel pump is a likely solution. Re-routing or insulating fuel lines ditto. A big aluminum radiator will usually help, and/or electric fans. Overheat problems frequently follow rebuilds, which may have resulted in thinner cylinder walls, and more heat transfer into the cooling system and carburetor. A phenolic spacer under the carb may help. EFI, with attendant high pressure fuel pump, and no float bowl to boil is the ultimate solution, but may come with its own problems. Lousy gas with ethanol causes a lot of the problems. In our area a 20 mile round trip is necessary to find unadulterated gas.