I have just acquired a '90 rs v-8 auto that is in great shape other that the tranny is shot. I have a couple 2004r's laying around and was wondering how hard is is to swap one into this car.

It should be somewhat straightforward, as long as your 200-4R has the correct bolt pattern to fit a V8. The shift quadrants are the same, so your shifter will work.

The transmission mount location will be different and it won't have a torque arm mount, so you'll need to deal with that. The easiest way would probably be to use a torque arm and crossmember from Spohn or elsewhere that will bolt right in. You'll also need a different length driveshaft.

Try to find the correct parts from a 89 TurboTA for the mounting and tailshaft. Good luck in that. But the Spohn parts are your best bet. And most 200R4s were a BOP pattern which is different than the SBC pattern.

The 2004r is in my monte right now behind a mild 350 so I know it bolts up. But I haven't worked on a third gen in a while so I forgot about the torque arm.

TTA parts are uncommon enough that they're not worth trying to source. Most 200-4Rs were BOP but some were dual-pattern.