The COE trucks have different cabs; notice the fender cutout on the doors.

With exception of COE all Chevy AD truck cabs are "the same size".
GMC pickup cabs are the same size as the big truck cabs up to a point (insert model number here). Above that they have "fat cabs" on the larger or "semi" trucks.

Chevy and GMC cabs are interchangeable and differences if any are very minor.

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-