First, why do you want to?

In reply to moving the spring hangers. I think that is a less desireable method in this case. The rear section of the frame has some shape at the rear spring hangers, tapers down for them, and in front of the hangers the frame is taller. This would affect the stance of the truck.
Another better reason not to shorten by moving the hangers is the frame has crossmembers at the spring hangers, these crossmembers help to distribute the load that is suported by the hangers.

The "proper" way to shorten the frame in this case would be to cut a section out behind the cab where the frame is straight.

I believe that for the AD chevy trucks if you want a 4100 frame you can't get one by shortening a 4400. The 4400 frame is made of thicker metal, and perhaps a taller section.

Again,
What are you trying to do, or accomplish?

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-