The company I worked for before retiring had for the A4LD, exchange bell housings. At a transmission seminar years back, it was pointed out that the A4LD pump bushing in the bell housing was align bored at the factory and just replacing the bushing could lead to problems. We first started out by just doing the customers bell housing, as we had the equipment do to this , but had such a strong response for this service, we ended up getting the various types of bell housings, replacing/line boring the bushing and having exchange bell housings on the shelf.