I see where your going.
option 1:
To convert from enclosed torque tube to open drive shaft you could go custom set-up, with axle and have a shop make-up a drive shaft and any tranny that will mate up to your engine.
option 2:
The 1st year Chevrolet Advance Design truck went with open drive shaft was 1955 1st series, you would have to find a 1st series truck drive train
axle, leaf springs, drive shaft (you can have one made-up), 4 speed transmission, you can change out your 4 speed tranny torque tube tail housing for a
open drive shaft tail housing you will have to find a open drive shaft main tranny output shaft and housing, it is more cost effective to find a used
4 speed tranny with a open drive shaft tail housing, I think Chevy used the same 4 speed tranny until 1960. So all you need is the 1955 1st axle and leaf springs.
The first year that Advance Design suburban offered a 3 speed on the column I believe was in 1947.
The E121 stamped on the side of the 3rd member stands for? All I could find is this
link It would probably be easier to fix your 1 3/4" off center problem especially if your doing a full off frame restore. That's my 2 cents worth.
Thanks
