I've had a couple of mid 70s Ramchargers and a Trailduster that had the plate, and my 80 1/2 ton 4x4 had the plate, but those were all small block trucks, maybe the difference?

I think, looking at everything your dealing with, I would enlarge the holes in the frame and add a washer under the nut. It doesn't explain where the issue rests, but it will move the project forward without causing a huge problem in the real life workings of the truck.

I suspect that at some point, someone will come on here ans say "I had that problem and ???? was the cause, and this was the fix". Then we will all be doing the head slap and go "OOOOOHHHH!!!!" But that might be a year from now or more.
Enlarge the holes and put it together. If the fix is identified, you can address it then if you wish. Gene