[Irish1941]

As was said, you'd need the 90-91 top
GM placed mounting bolts right into the top's inner steel structure. It would be pretty hard to do it yourself and be safe.
You would also need the 90-91 center belts as well as GM switched seat belt suppliers and the 78-89 belts won't work with the 90-91 shoulder harness.
Plus think you need to add the outer male seat belt bolt mount to the floor next to wheel well.
73-77 belts bolted to the floor thru the rear leg of the seat if I remember right.
Only real problem after that I can see is the armrest of the seat getting in the way of the shoulder harness.
Might be able to use a 87-89 armrest?

As for the switch to lap belts, unbolt the 3 point floor mount and bolt in the 78-89 male ends and use the matching 78-89 center belts.
The shoulder harness uses same hole beside the seat in the floor as the earlier trucks.

oh and the later top bolts are just torx T35H or T40H.
Any parts store should a set of security torx bits.