I've cut out part of my firewall and measured it to replace some, it's 16 gauge. not sure about the rest of the cab but I expect it to be the same.

When you order your sheet metal ask for cold rolled, it works and welds better than hot rolled.

•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-