I’m happy to report that it appears the mystery has finally been solved! And so what
was it?
After weeks of trying virtually everything, we were just never able to identify the real problem and resolve it. In sheer desperation, I finally turned it over to the fellow many of us call “The Guru” here in our local club in Jacksonville, FL. H.L. Chavin would call him a “Model A Whisperer” with the patience and know-how to sleuth out the problem.
According to "The Whisperer":
The grounding strap from the battery to the frame was not always providing sufficient ground to provide ample and consistent juice to the distributor. Sometimes it worked just fine—like on my long drive to coastal Georgia where the truck ran fine for 110 miles before it suddenly started acting up—and then other times you couldn’t even get the thing to start.
The local whispering guru says a continuity test with a meter confirmed this.
Since this thread was getting a little long in the tooth, I posted a much-more-detailed report in a separate, updated thread which you can read here if you like:
http://www.fordbarn.com/forum/showthread.php?t=100328
Again, thanks to all of you for your time, suggestions and most of all, your patience! I sincerely appreciate it.