The transmission crossmember which the torque tube ball collar support bracket,mount bolts to, is bolted to the frame (not riveted), on a 3100. I am guessing that is why it isn't included in the factory pictures of the frame. If you are running a torque tube rear axle, you need to find a crossmember and the torque tube ball bracket/mount.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission