Any GMT400 bench will bolt in. No need to drill holes. Actually, the opposite. Have to plug the inner holes from the buckets next to the tunnel.
You could grab the middle set of belts from the donor, but they are ponied up with the latched ends for the outers. I'd get the inner set & the outer latched ends which come through the seat from the donor. They are encased in a plastic cover, grouped together. It'll make the install easier. If you can't get those latched ends that come through the seat from the donor, you may (may!) need to use a longer bolt to attach them to the floor. If you get some that are the same color as what's in the truck now, the outside belts that attach to the B-pillar won't have to changed. That is, unless it bothers you that they are a different color.
Early benches, pre-'95, are sooooo much more comfortable than the later benches with the three scooped backrest sections! The later seats have to be the most uncomfortable ones ever designed by GM! The '88/'89 benches did not have headrests.
Do not, do not, do not use the bench seat with the little driver side flip down armrest! Absolutely no!!!
If you find one of those, you MUST send it to me!
Consider that the cost of this info! LOL!!!
Or...
If your son REALLY likes the bench, (he will!) and you find that storing those buckets is a Royal PITA, 'cause dammit, they're just in the way, I'd willing to offer FREE storage for as long as required, for the one time charge of the shipping costs to me!
Gotta remember though, Canada Customs agents are 'power trippers' & they can be fussy about things like this leaving the country, so it may take a while to get them back to you!!!
