I don't think the harness thing works without the seat to match, because the ends would slip of the edge of your shoulder without the slots in the back of the seat to keep 'em in place.
When I was looking at keeping my stock '91 high backs, but I wanted to run a 4 point harness, it was not recommended by the couple of people I talked to. Your stock belt works because it runs across diaginally from shoulder to other side of body hip. But a 4 or 5 point harness is coming at you from rear, not the side, and running down to your crotch. So in normal driving where the belt isn't sucked up against you super tight, the belt can have a tendency to slip off over the edge of your shoulder. I bought my 4pt harness and tossed 'em in with my stock seats just to see how they fit. Indeed they were right, bought some corbeau high backs with the slots in the back, and am good to go.
I'm running a cage, btw.
Without a cage, where are you going to mount the points to. You have three (if your truck had shoulder belts), ok, how about the rest? The high point over the right shoulder (for driver) seems like a tough one.