Gents, Just a technical note to help the conversation move along. wink Original Chevrolet resource info I found for 1958 Chevrolet Pickups lists all 3100-3800's came equipped with a 17-1/2 gal capacity fuel tank in cab. The 18 gal capacity indicator comes from the descriptions given to reproduction replacement fuel tanks by most every vendor such as the Tanks Inc Model TM55B.

You can see all the fuel tank capacities in this 1958 Chevrolet Truck Full Line pamphlet. Anyhow, moving along... grin
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)