if using a shoulder belt and you are looking for a possible mount point you could look at some newer vehicles with an adjustable shoulder mount point. these attach with what boils down to a rail that attaches to the cab with a couple of bolts and then the shoulder belt attaches to that with a height adjustable part. it will be tough to get behind the corner sheet metal but possibly a preformed bracket with the nut(s) welded to it and a short stud threaded into the nut(s) could be fished up using a wire that goes through the attachment hole and then is attached to the stud. draw the stud up through the hole, hold it with some needle nose pliers while you install a flat spring clip to the stud to hold it in place until you can install the rest of the parts. remove the wire after the part is in place and held with the flat spring.
here os a link to an example of a spring clip retainer. its for a brake rotor but you get the idea
https://www.idparts.com/rotor-retain...30-p-6101.html