Doug,
Did this on a friend's 64, pretty sure the back half of the cab is the same, though.
Drill a hole just big enough for the bolt that attachs the shoulder point to the cab. Then we welded a nut in the center of a piece of 14 or 16 gauge metal about two inches by three inches. Although before we welded the nut to the plate we contoured the plate to the cab corner. we then put the plate and nut up behind the cab corner until the hole and the nut lined up. We put a bolt through the nut to hold the assembly in place. Then we drilled four 1/4" holes so we could weld the nut and backing plate to the cab. A little grinding and it looked almost factory. All you see from the can is the hole with the nut behind it and that gets hidden once the shoulder point is installed. We used belts out of a 95 S-10 and the retractor actually mounted about 18" off the floor, right where the fuel tank hose used to be (we also moved the tank under the bed, that's another story.)
It hasn't been involved in an accident so I'm not sure if it up to OSHA standards!
Unless you like aggravation and explosions take the seat and gas tank out of the truck.
Don't have pictures but I'm going to do this to my truck in about two months, I'll post some pics on this thread when I do. I'm going to use the late model s-10 belts as well.
Good luck