I can't tell from the pictures if you have 6 lug stud centered "Budd" wheels, are your rear wheel nuts two different parts?
Of do you have hub centered wheels?

If stud centered Budd wheels you have lots of newer (but not as many brand new) axle options. If hub centered "Motor Wheel" then you have limited axle options (only old stuff).

In any case do some digging and see if you can find the faster gears for your existing axle. That will be simpler, lighter, more reliable, and cheaper than installing the Mitchell.

I don't know what to tell you exactly to look for.. If you start calling junk yards someone will eventually know what you're talking about and help you figure what model axle you have and what center section you need, possibly interchange from even newer or other model Eaton axles than what your truck came with.
One of my favorite Junk yards for old and newer driveline components is AD Weddle in Roanoke, VA, Mike might be able to help you out 540-345-2331

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-