Just make the Vortec's original harness into a standalone by eliminating everything that isn't engine or transmission related. You should end up with a couple grounds, B+, and ignition to connect to the 89's electrical system. Be sure to fuse the 2 hots.
Nothing in the interior is run by computer in 89. Even the speedo which connects to the TC
is independent. Also the 89-98 use 4wd use the same VSS. You may have to get a different TC adapter for the 4L60E. They used 4, 5, and 6 bolt adapters over the years. I don't think there's a way to make the 700R4 work with Vortec because the TV cable will have nowhere to connect to and it is the transmissions carotid artery. W/O it the transmission will die a quick painful death.
You could check with Bowtie Overdrives. If anyone has a kit for the Vortec/700R4 it would be them.
I don't see a kit listed on their website.
You would also be better off with a transmission that was designed for your engine and has 9 years of improvements.
I personally would only use a 98 4L60E and PCM because the lockup was upgraded to EC3 in 98.
Before you put it in it would be a good time to upgrade it a bit for reliability and longevity by a few inexpensive changes like clutches, bands, sunshell, sprags, and a 5 pinion planetary. Be sure to use the GM 24227002 & 24218069 5 pinion. Not some Chinese import disguised by a seller on Amazon or eBay. You'd be better off with the 4 from GM in that case.