I'm wary about exchange boxes having been caught out with a Mazda Xedos autobox. Beautiful thing when it was working but £3,000 plus to replace with Mazda.... I got a supposedly low mileage Jap import box instead and a local garage to fit it. It tied up one of his lifts for nearly a week and he was so glad to see the back of it. A month later, it displayed similar faults to the one we had just removed. But the warranty on the "new" box was pretty useless, invalidated as the garage had to modify the box to fit the UK car, and even if I had won that argument, there was the hassle and cost of getting the thing back out and of returning it to the supplier. Turns out the autobox was the one weak link with the Xedos so I part-exed car for a manual. (Still have it, 15 years old, problem-free and no rust after 180K miles. Not going to be saying the same about the Alfa....)
It was only after I had fitted the exchange box that I was told about Autotransmission in Gloucester. They would have rebuilt the original better than new, replacing the weak parts with uprated bits. Would probably still have that car now had I gone down that route.