Later, I decided I wanted more than just lapbelts. After I installed my roll-bar, this became a practical idea. I do not believe you can safely install a shoulder harness in a Scout II without a roll bar. There have been instances reported where a Scout II has rolled, and the hardtop has come off (or sheared or..). If you're shoulder harness was connected to your hardtop... let's not think too hard about this one.
Now that I had a rollbar, I started looking around for shoulder belts. It just happens to be that I had a 1973 Buick Century Station Wagon in the backyard that needed to be gutted and hauled away (a remnant of my mom's hot-rod station wagon period). The '73 Buick (and the '73 Pontiac wagon we had) uses the old fixed position two-piece 3-point harness. That is, after cutting away the plastic, the shoulder piece of the belt is bolted to the car's upper body stucture with no ratchet mechanism. The webbing is anchored around a short piece of steel, with bolt holes in either end. The lap belt portion is a standard GM lapbelt, with an "eye" in the male attachment that the shoulder harness snaps into. This means you can use just the ratcheting lapbelt, or you can use the lapbelt in conjunction with the fixed length shoulder harness (leaning forward to release the parking brake can be fun if you set your shoulder piece as tight as mine!)
With the scavenged seat belts in hand, I went to work. This is the easy part. I purchased a small piece of flat stock steel. It's probably about 2.5" or 3" long since my rollbar is 2" tube. I drilled two holes, one on either end. I then purchased some Grade-8 bolts and associated hardware. I think I used 3/8" (it would fit through the seat belt's mounting piece).
On the floor, just replace the stock units with the GM ones. You'll need to buy some Grade-8 washers, since the IH belts have smaller holes in 'em than the GM hardware, so the GM belts could slip over the mounting bolt (or at least I didn't feel comfortable about it). The stock male attachments are fun to get out.. the plastic wouldn't budge on mine, so I torched it (almost cried). Get the old males out, and put your new males in. The Buick's male ratchets were angled, and didn't quite fit nicely. I had to wedge 'em in (they press up against the seat a bit) and then tighten down. Works fine now. The female pieces are a snap to bolt to the floor.
The shoulder pieces were placed against the front of the roll bar, (I think) above the Y in the legs. It's important to have the belts at least at shoulder height. You want 'em a little low so you don't slide out of your seat in a roll over and get between the rollbar and the dirt, but if you put it below your shoulders you risk serious back problems in an accident. Anyhow, put the mount plate in front of the 'bar, put your piece of steel stock behind it, and put your grade-8 bolts through. You've just made a type of home-made U-bolt. I already thought of using regular U-bolts, but they don't make 'em with the right angle to go around the rollbar and still meet the holes in the shoulder belt mount. You really have more of a trapezoid shape than a rectangle or square.
With my shoulder harness fixed attachment, I can snap the shoulder piece into my male ratchet piece and ride around safely, or just use the lap belt in those not-so-scary trail sections. OTOH, my fiance' hates using her shoulder harness, so it works out well.. she doesn't have to tuck the belt under her arm.
I know GM used this style in '73, but my '77 Monte Carlo has the convential style we're all used to seeing, with a ratchet mechanism that operates both the shoulder and lap belts as one piece. I've seen this style used as well. Simply replace what I described for the upper shoulder mount with a D-ring like slider for the harness, and make your own U-bolt type mount to the rollbar.
A few personal notes: When someone else isn't in the truck, it's helpful to either hook the shoulder harness into the lapbelt, or wrap it around your rollbar.. otherwise, it swings when you brake or gas, and clangs against your rollbar. Along those same lines, I personally like the fixed attachment shoulder harness because I can disconnect the lap/shoulder belts, and rear seat entry is a cake, since the shoulder piece just hangs down to the step in the bed. With a ratchet style, you have the whole ratchet mechanism and all the belts that'll be in the way for the back seat riders.
As always, Your Mileage May Vary.
Here's one pic of my roll-bar mounted shoulder harness. The other photos didn't turn out very clear (cheap-o $35 pawn-shop Vivitar.. but, if I drop it in the mud.. who cares!). As you can also see, this shot is taken at the very beginning of my "tear-down" of the '72.. the front and rear bench seats have been removed, and I'm about ready to tear out the rollbar and then the flooring materials.