I considered this years ago and even gathered the parts to do it. I had (still have unfortunately) a core tranny and a TCI adapter. It never got out of planning/parts gathering stage because at the end of the day, the rebuild and mods to get it to hold up to my motor..plus all the fabricating.... it wasn't worth it.. for me it wasnt

your tranny doesn't need a rebuild so that's good but unless you start changing some hard parts (not just rebuild items) be careful not to put too much power to it or sticky tires on it.

your 89 trans has the aux valve body so that's one improvement over earlier ones that already have.