I would advise against using a stock used 200R4. They are usually junk. They were junk when new. Not going to hold up with any hard use at all. These trannies need to be beefed up to handle a strong V-8.
On the other hand, since I believe you have an OHC6 the 2004R would be acceptable. These transmissions were used behind V8 Olds and Turbo Buicks. It should hold up fine in this case. The first gear is deeper than a TH350 and the overdrive is nice. If you're going to the effort of a swap, this would be the way to go.
