Something comes to mind, first: I doubt the generator's voltage regulator swap is causing the problem, but they may have disturbed something or removed a battery lead from something and didn't fully tighten a bolt...
GROUND LUGS. Check them all by removing them, cleaning them, tossing the old corroded bolts & washers, replacing with new. You might need to do this on the engine to frame ground lead as well. Start near the generator where the work was done. Also, check out the battery compartment very carefully. Check all the battery connections. Follow the grounds to the frame and check that the bolts are clean and tight. Naturally, check the positive connections too.
BATTERY SOLENOIDS: That old of a rig, you might check them out. And all their electrical cables and connections. Next time you take a drive, carry a rubber mallet with you. And if the problem occurs, whack those solenoids, see if it starts up immediately. If it does, replace.