Transmission

First off, Welcome to FABO!
Second, as stated S6 and V8 have different bellhousings.
Lastly... Good news is you have so many options, especially if you're bolting in a Smallblock V8. You can get what you need from any V8 car, Van, 2WD truck (both fullsize and the Dakota's even the V6 ones) up until mid 90's for automatics. Manual tranny's from the later trucks have that "granny gear" that won't make it optimal for use in these cars.