This worked for me for about 25 years. Don't ask me how they did it. Changed over from an early Ford Box (39?) to a Cad Hydro. I know they shortened the Ford torque tube and the inside shaft was shortened by cutting it off, grinding the end to the inside dimension of the coupler and welded it in. The back of the Hydro was modified to take the torque tube coupler. This is it out of Chassis #1 and installed in Chassis #2 of 3.
I seem to recall seeing an article maybe in HotRod where they used a Buick u-joint that fit the hydramatic splines on one side and the Ford splines on the other.
If ya go open drive***, the rear bones on there own,with out Torque tube* are not strong enough,and will bend with any real HP n bite.= The torque tube was/is the strong part. So add bracing to rear bones. if open drive....