Sometimes it has nothing to do with being "hot" and sometimes it does. Try running with gas cap off during next trip. Could be sucking a vacuum after awhile.
As far as the heat thing.
1. You say it turns over just as fast when hot, Right?
2. Not sure if "glass full" on pump means it's completing the pump to carb. Next time squirt gas down carb throat to see if it starts. This will identify or eliminate a few things.
3. Possible heat soak, boiling gas out of carb bowl.
4. Seems like you are saying you have a 12V system with a ballast resistor. That is correct for 12V conversion. It is a white ceramic block with one wire in and one wire out. Should not affect starting if bad. Should have two wires to the coil positive. Right? Foot start or key start?
5. Is your GMC positive ground?
6. I want to blame the starter or its wires, but only if it drags or is slow when hot.
7. Be sure that whatever type of choke you have is not closing due to heat.
8. Do you smell gas after hot shut down?
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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