keep the torque converter? (360 swap)

I just went through this and I would say don't use the donor torque converter or donor transmission. I bet your donor is a 727 and your old one is a 904 right? So not only will the 904 be better in many ways the stock 904 converter is a higher stall most likely. You do need the b&m externally balanced flexplate but you will save that much not having to shorten the driveline or buying a new yolk.