Friday, I removed all the dash panels so I could see the entire length of the harness. Where the harness runs up the front bulkhead, I saw that it was pushed tightly against the bottom mounting bolt head for the control valve lever bracket-left side.
It appeared that the bolt head was buried deep into the tape wrapping the harness, and I was certain that I had located the problem spot. No such luck! What the bolt head was buried in was a 3 inch section of sponge/foam type over wrap covering the loom at that spot.
The wiring inside was intact.
I continued to open the harness and inspect the wiring.
Thus far I have cut open the entire length of harness except for the section that is stuffed into the white plastic hanger channel behind the brake pedals.
I am re-taping the harness as I go about every 8 inches (so as to maintain the OEM shaping somewhat).
I have also checked the connector bodies where the front harness mates to the fuse/relay box harness and those are good and clean.
I removed the one zip tie and the saw tooth clip that secure the wire bundle to the underside of the fuse box and spread all those wires apart for a look.
Absolutely nothing wrong.
I then got out my Ohmmeter and tested the ignition switch resistance.
The switch that is in this truck has a metal body and only three terminals. The harness connector also has only three wires.....White-blue power in, Black-yellow power out, and Black-white starter signal.
Resistance across the White-blue to Black-yellow terminals is as high as 9.4 Ohm.
Testing across the same on a brand new switch shows.3 or.4 Ohm.
I know the electrical load on the White-blue and Black-yellow wires from and to the fuse box cannot possibly be all that great because there are no really big individual loads (based on each fuse ampacity) fed by the Black-yellow wire.
On this truck (non TWC) the electrical loads present when the switch is in the RUN position are:
Ignition coil and igniter module, main fuel solenoid and its relay, temp gauge/sender, transmission range solenoid, and alternator "S & IG" loads, and the engine shutdown timer and relays.
I am wondering now if perhaps this ignition switch resistance might actually increase under a load to exaggerate the amp draw that the 40 amp AM1 fuse has to bear but the individual branch circuit loads don't blow any of those fuses because the summary ampacity of the branches is great enough to stave off thermal overload long enough for AM1 to fail (which then interrupts all the branch circuits too).

As to the starter relay being involved in this somehow:
The White-blue wire indeed provides the power that will activate the starter motor solenoid, but there is a load draw on that portion of the White-blue circuit ONLY when the key is turned to the start position.
So that load would not be present when the key is not at start position. The only way a defective starter relay could could be adding to the RUN load on the White-blue circuit would be if the W-b input of the relay were somehow bleeding over to the relay coil ground wire terminal, and I don't see that as a likely scenario and still function properly otherwise.

I plan to open and inspect the remainder of the harness on Monday even though I suspect it will reveal nothing, I just can't chance it after having cut open the rest of the harness.
Finding no culprit I will restore everything to running order and install a new ignition switch. That metal bodied switch has my suspicions raised as this is the first metal bodied ignition switch I have found on a 7F.
This truck was transferred to our dock from another city where it was maintained by either a small shop staff, or outside vendors. Because that terminal was closed up, their small fleet was redistributed nationwide and I have no way of checking any real time history.
I did get 1 clue that this problem has been around a while because the OEM 40 amp plug fuse has been replaced with a short 2 wire ATC fuse holder.
I even wondered if possibly the plug type fuse would be better able to resist blowing if the overload was an expected even by design of the system. We tried an OEM plug fuse and got the same results as with a 40 amp ATC.
I will definitely update this with my findings as I continue solving (hopefully) this issue.