My 48 has the inner sheet metal where the tank would have been. The tank was mounted outside the frame.
In smaller trucks, same year, the tank was inside the frame.
I think 1949 is the year the tanks came back in the cab. On the other side of the coin is late 47 and most if not all of 48 the tanks should be outside of the cab.

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-