I just did a SM420 install after removing the 3 speed on my 53. It has the torque tube enclosed driveline. I kept the cross member and the rear transmission mount. The assembly manual does not say whether a cross member and mount was used or not on the 3100 torque tube. Are you guys saying there was no transmission crossmember or transmission mount used in the optional 4 speed on 3100 trucks with torque tubes?

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration