Personally, I'd say drop another 700R4 in, though preferably from a 90-91 year model. They have upgraded internals over the older ones from what I understand. Make sure, though, that you don't get a 4L60-E, as those use the truck computer to control shift points, which the 89 model doesn't have. Also, make sure you get one from another 2wd, since the tailshaft is different on the 2wd and the 4wd. Other than that, if you wanted to build it up with upgraded components, it should last him well.