Why is your truck running so hot?
Maybe it is running too lean after you switched from FI to a carb?
Maybe the timing is off, retarded timing may make the engine run hot. Too much advance may make it hard to turn over.
If the starter locks up because of engine heat, try to get the engine to run cooler. Try to cure the hot engine problem first.
Did you change the distributor from a FI one to a regular one that came on an engine that had a carb?
Check the timing mark on the balancer against top dead center on your engine.
I don't mean to step on anyones toes, but maybe if the engine was not running so hot there would not be a starter problem
I like the idea of a Ford solenoid, maybe it's not necessary.
I have a 454 in my CrewCab Dually that pulls a 15,000# gooseneck trailer, if anything would get hot , that would. We have never experienced a starter lock up. The engine has over 200K miles so her tongue is hanging out after a long haul. No starter problems.
Just a thought and my .02
CCDually