The swap from the wrx to the sti is pretty easy. You need to get an automatic tranny driveshaft from the same year subie for it to fit.

If not, a wrx isn't going to handle a good amount of power to the tranny. About 260-280 i've always read is the limit for the stock wrx transmission.

But then again, it doesn't sound like the cars that you're talking about are going to be faster than 260 at the wheel, so you shouldn't have to worry.