Frame width on a 52 6400 is 36"

Have you checked the disc brake sticky thread and the P30 tech tip in the big bolt section of the tips?

I have used an 05 freightliner rear and suspension on another truck, brand new housing, brakes, suspension, and used 3.73 gears, didn't pay anything near 2500 for it and it came from a regular big truck junk yard. It's a pretty big axle to go in an AD Chevy, a Rockwell meritor rated for 12 or 15 K lb.
If you plan to adapt a matching front axle it'll be difficult because they'll be so wide.

Have a look at the P30 stuff.

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-