i swapped a 4r70w in my 91 a few years back, its been a while but it was simple all plug and play, i used a 97 cougar shifter, it bolted right in and i had to massage the console a touch but the cover did manage to fit around it
here is all the details you will ever need to know about the swap
Ok,...Heres another question. ..Do you have to modify the shifter cable bracket that attaches to the tailshaft housing???..It looks like the OSS sensor is in the way and the little 8mm headed bolt on the top has no threaded boss..