Frame rails are 36" outside to outside and begin to taper in part way under the cab.
The cab is about 3-4" wider than the frame at the back edge where it nestles around the frame.

You should also measure the width of the front axle/tires and compare that to original, the newer model truck will be wider and even if it might just fit it will look a little strange with tires trying to get out of or already escaping the fenders.

•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-