Not sure why you're worried about rewiring. Your '89 K1500 is going to have a 700R4 (4L60) so the only wiring you have already is for the torque converter lockup. You have an ECM not a PCM so it's not doing any transmission control like a 1993-up with the 4L60E.
The only wiring the manual trans has is for the reverse lights.
For GM, 1992-1994 NV4500's have the old style external slave cylinder with fork type release bearing setup, and a lower 1st/reverse gear set (6.34:1)
I think 1995 may be a one-off year (as many GM truck 1995 pieces are) with the newer gearset but may still run the old slave cylinder setup. My memory is a little foggy on this.
1996-up for sure is the new setup with 5.61:1 1st/reverse (Dodge style) and has the integrated slave cylinder/release bearing setup that slides right over the shaft and bolts to the front bearing retainer.
Be sure you can fully inspect any NV4500 before you buy it. They're tough as nails but do suffer from a couple problems; the synchros from 1st-2nd often go bad and once it gets operated with those pieces floating around inside it usually ends up needing a full rebuild. People also use the wrong fluid in them because the correct fluid is only available at GM and Dodge dealers; it's re-labeled Castrol Syntorq LT and must be used to protect the carbon-fiber synchros. If it's been run with the wrong fluid...oh well. It'll need a rebuild. Rebuild will set you back a grand easy if not more, might as well do the clutch kit while you're in there and don't forget the pilot bearing/bushing for your crank. Also, the original carbon-fiber synchros that made the NV4500 shift somewhat smoother than a typical heavy truck transmission, are no longer available new, so you're gonna get regular bronze/brass(?) aftermarket pieces that won't shift as smooth. Don't be tempted to run different fluid at that point though; I'd still stick with the Syntorq which will set you back about $100 for the 4 quarts needed. GM part # 12346190 you can find it cheapest on Amazon.
Richard