Sorry for taking such a long time on the update. I just purchased a house and am trying to knock down a wall and remodel three bedrooms into two. As you can imagine, my time is a bit limited at the moment.
Well, after MANY frustrating hours I was able to isolate the gremlin in my fuel system. It turns out that there is a fuel reservoir plumbed into the lines that allows the truck to continue to draw fuel while switching tanks. It ALSO has a check valve that (I assume) was put in it to keep the fuel from draining back to the tank while the truck is turned off. That check valve was intermittently faulty. It would sometimes actually block fuel from getting TO the engine instead of blocking it from returning FROM the engine while off. It was also completely unnecessary because when the truck is off, you can't get fuel through the high pressure pump anyhow. So the pump acts as a natural check valve.
So after dismantling the entire fuel system TWICE and testing each component (including the lines), I was finally able to find the culprit. So I simply pried the check valve out of the reservoir and reinstalled it. Runs great, even after sitting. So it has no issues with the fuel bleeding back to the tank. It holds decent pressure overnight.
So if you ever have issues like I've described, look for the reservoir. It looks more like it should be a filter from its exterior. In fact, I believe the '86 model has one that actually had a filter in it. But that was the only year that did. Other than that, it is just a fuel reservoir and check valve system.
Thanks for the compliment, kroussinoffroad. Trust me, it almost DID have a 302/C4 combo in it. I had the engine, trans, and transfer case adapter purchased and sitting on my garage floor. I had every intention of rebuilding the motor and installing it. But then I would also have to buy a new radiator, I would have to buy an 8.8 and have it rebuilt with 4.10 gears and a limited slip (because the 7.5 would never hold up), etc. I decided that I just didn't need that much power.
In the end I sold the package and kept the stock 2.9L. I actually like the 2.9L. It's a decent motor with no major defects. In fact, I have a 1986 parts truck I'm going to pull the motor from and slowly rebuilt it. I'll use all the tips from Pat Kunz that I've printed off and saved over the years (Kunz's corner) and turn it into a powerplant that should surprise a lot of V8's.