To install later model rear cab shackle mounts on 47/48 cab requires some special shaped sheet metal reinforcements or braces, possibly available as repair parts.

To install 47/48 cab on later frame it either bolts on with the original 47/48 center mount, or two 3/8" holes need to be drilled in existing frame crossmember at back of cab then it bolts on..

I thought 54/55 had rear cab shackle mounts like 49-53? The assembly manual pictures you reference with different mounts are they for a panel truck or suburban?

•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-