Hy prubah, that transmission is a sliding gear transmission that has nothing in common with the SM420. The bellhousing bolt pattern is not correct to allow bolting the SM420 in place. There is no factory bellhousing which allows the SM420 to bolt up to it and provide the mounts for the clutch and brake pedal shaft. While there are SM420s for torque tube driveshafts the SM420 is a longer transmission than the earlier sliding gear transmission and will not fit in place of the sliding gear transmission. I'm not saying the swap is impossible but it is not a drop in proposition, hope that helps.
