That is the same crossmember I have that was used on the three speed, but it also had the rear mount attached to it. See this:
https://www.classicparts.com/1947-54-Rear-Transmission-Mount/productinfo/79-204/It retains the rear two bolts of the torque tube to the mount and the mount is attached to the cross member so it does have a function other than to rest the torque tube when the trans is removed. I am inclined to think the sm420 used the cross member and rear mount as well to support the torque tube otherwise all the load of the driveshaft will be all on the four transmission mounting bolts themselves.
