OK, so far I can't find the length of the different models in my books, but have a few more to check...

Looking at the pictures in the manuals it probably is a Spicer 6xxx or a 7xxx. They have the same sort of mounting ears as seen on yours, and a 5831 is defiantly different.
A 7xxx Spicer is really big, probably looks like it should not even fit, a 6xxx Spicer could look at home in your truck...

There really should be a tag on your transmission, and if not then you will find the two rivet or screw holes where it is missing. Try looking about 1 or 2" below the top on the side near a corner, start at the left front and then left rear.

Grigg

•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-