Is the resistance wire for the coil still in the circuit? If someone has rewired the truck, it may have been removed. This is typically done if an aftermarket electronic ignition is installed, but if your '72 still has the original system (points, condenser, and coil) and the points are closed when the engine is stopped, the coil and wiring can overheat.
More importantly, the ACCESSORY position is NOT the same as the run position. ACCESSORY is activated by turning the ignition lock cylinder backward, not clockwise.
Further, is it actually the ignition wiring (for coil power) getting hot, or the orange wired accessory circuits? By your description it sounds as if the accessory circuits are overheating. That could be any number of things from the alternator regulator (mechanical) to the body accessories. If you start pulling fuses it may help isolate the problem areas.