[yossarian19]

Heh, I'm a pro mechanic but almost none of this project is stuff i'd done on the job before. I think we're (most of us) on a level playing field in this hobby, except the guys who work in body shops or metal fabrication.
Thanks for the kind words all.
I haven't had time to work on the truck but there have been two promising developments.
One, I finally got a quality brake flare tool. It's a world of improvement over the $30 cheapie I'd had before.
Two, I found a way to mount the shoulder belt that'll work for me.
What I'm going to do is this:
When you open the door there is a little rubber bumper in the upper rear corner. Pull that bumper, notice the small hole that it inserts to. Right about there is where I'm going to put a 3/4" hole, through both sides.
I'll cut a bit of 3/4" outer, 1/2" inner tube and run it through there, cut flush to the sheet metal on both sides. Weld in place.
1/2" bolt & nut the shoulder belt, add some RTV for weatherproofness & a small adhesive backed rubber bumper for the door.
A 1/2" nut fits just fine in between the door sill and the door. I'm not *that* excited about how it will look, but I couldn't come up with anything else safely inside my metal working skill.

The other option, requiring tools I don't have & skills I'm unsure of, is to cut a flap just above where the B pillar widens out, put your anchor plate back there, bolt the seat belt in & then re-weld that flap. This is probably the best overall option, but as I'd said - tools & experience.