I have the same problem with my 55 2nd with Pontiac V8. The bellhousing is a full circle one with out removeable bottom. Some day when I need to replace clutch, The stock crossmember is in the way of removing the bellhousing. My thought is to try and remove rivets from the crossmember and replace them with bolts. Don't know if I can reach all rivets or if crossmember will be able to come out from the bottom. I think a couple of the rivets are common with the rear spring mounts. I have thought about cutting out enough of the crossmember to allow clearance, but I'm not sure that wouldn't compromise the integrity of the frame. The Pontiac V8 has mounts at the midpoint of the block, which are setting on a fabbed up crossmember, and the Muncie 4speed is also setting on a crossmember. So the stock rear crossmember bears no weight, but it does attach to the frame rails at the spring mounts. And it is a heavy duty gusseted part of the frame.
