Probably this comes after you've got it all done ("NOW I tell you!) but in case it will help you or someone else, here's the rear axle removal procedure from the 40 Buick Shop Manual, likely similar to your 38.
1` Lift rear end of car with hoist.
2 Disconnect parking brake at equalizer.
3 Disconnect hydraulic brake line at fitting anchored to left channel iron strut.
4 Disconnect rear shock absorber links at lower end.
5 Disconnect rear radius rod at axle end.
6 Disconnect rear chassis springs at lower end. Screw retaining spring at lower end is left-hand thread. Tie lower end of springs to rear bumper with wire to allow working clearance and prevent damage to hydraulic brake lines.
7 Disconnect front end of torque tube from driving ball.
Only seven easy steps, right? How hard can it be?
The transmission section of the manual doesn't cover removal - but the drive shaft spline goes about 2 3/8" into the transmission, and the gearbox input shaft spline goes about 7 1/8" into the bell housing/crankshaft. So to remove the trans with the engine in place is going to be a challenge, I think. It looks to me like the torque tube would have to be back almost as far as the point where the X frame sections meet, in order to have room to pull the trans input shaft all the way out past the bell housing. No doubt there are designs more convenient to the service and repair trades. My engine was out when I waged the (several sessions) battle to separate the trans from the torque tube, and I just outlasted it today. The 40 shop manual says the engine and trans are to be removed as an assembly.
Stephen