Yo!
I haven't seen anyone do this yet. As you know, most add the roll cage so they can attach the harnesses.
BUT! how about seeking a 92-96 camper top (as Steve once wrote, "LOTS o' work") w/integral shoulder belt/ mount & rear bench & using that in your 89? and you'd need to hook-up the 3rd brake light, but that's no biggie
here is a diagram by Ford via Steve83 that shows the rear bench belts in 92-96 in case you decide to go w/a jy top/bench
http://www.supermotors.org/vehicles/registry/showmedia.php?id=254220
Speakin' of Steve; just found this in my MS WORD files; haven't had a chance to add this & other info to my site yet;
"...You can buy a good used '92-96 camper shell for $20-100 any day of the week to get the hard points in the roof. But the rest will be a little more difficult. Finding good rear shoulder belts from those trucks is harder - I've only found 1. Then you'd have to drill & weld in 2 more hard points to mount each belt: 1 for each reel on each wheelwell, & 1 for each end on each side of the rear seat. Then you have to swap in a '92-96 rear seat, or at least '92-96 buckles. Doing it with a roll bar wouldn't be much easier, even though you could use ANY roll bar & just have some nuts welded in to recieve the shoulder bolts. You'd still have to come up with a belt system that you'd trust with your life (or your family's). I'd stick with using factory parts because you KNOW they meet DOT specs for rollover safety..."
