Intermittent connections are obnoxious to troubleshoot.
It sounds like there's corrosion or a loose connection that "magically heals" itself when you apply enough current to temporarily blow through the corrosion. These are miserable faults to locate but it can be done.
The J-Studs should be on a diamond shaped piece of thermoplastic between the transmission tunnel and the brake booster. I'd remove the ring terminals from each stud and clean em up even if your problem child isn't performing right now.
This guy has an 86 with the J-studs near the top of the firewall. Circled in RED. Yours' may be lower down or on the other side of the tunnel but all the squares had em.
Buy or borrow a remote starter button with alligator clip connections so you can clip onto the starter solenoid and bypass the ignition switch without being under the truck when this intermittent crops up again. O'Rileys has a Lisle unit for less than $20. If she starts right up look at the Neutral safety switch. If she just cranks then you have a problem between the supply side of the ignition switch and the BAT stud on the starter.
Just curious how you load tested the battery?
I have a big ole heavy sheet metal battery tester with Ni-Chrome heating elements inside and a momentary toggle switch on the bottom. When I connect the clamps to a battery and switch it on it instantly draws a huge amount of current to heat those elements. The big analog meter on the front shows the charge level before and after switching the elements on for several seconds... IIRC a test cycle is around 10 seconds... It says how long on the tool. If the battery is weak it won't bounce the meter back into the green area. You can't rely on a test done immediately after you pull the battery charger leads off or switch the truck off after a drive. A weak warmed up battery will usually lie to a load test right after you pull the charger or kill the alternator. I usually let a battery settle down for at least 12 hours before I hit it with the load tester.
A couple more quick questions....
Are your battery terminals molded onto the cables or are they the strip and clamp service replacement type?
Side or Top Terminal?