When I did the swap in my 68 Barracuda the threes speed bell housing didn't have the bolt pattern for the 4spd and wasn't deep enough to drill and tap for it. If the mounting surface on your bell doesn't extend below the 3spd housing and have an extra set of threaded holes below the 3spd then you will need a new bell housing.
I used the A833OD 4sp from a \6 81 D100 pickup. That bell housing had both patterns. The truck A833OD has a longer tailshaft than an a-body but otherwise fits. You will need to fab an adapter for the shifter to have it come out in the correct location and you will need to shorten the drive shaft. The bigblockdart.com page previously mentioned has a whole right up on using the long tailshaft tranny in an a-body.
What ever tranny you use will also require the slip yoke from a 4sp or 727 car. The output shaft is larger than the 3spd or 904.
