Does your transmission have after it an enclosed driveshaft (I think so)?
If you want to use a rear axle or center section as 3B suggest with open driveshaft you need transmission with open driveshaft.

If enclosed now you can swap some parts around on the back of your transmission from another SM420 with with open drive line to allow use of an open driveshaft. Or swap entire transmission.

Last edited by Grigg; 06/06/2017 9:21 PM.
•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-