It seems odd an overload spring accessory is not listed in any of the Chevrolet Master Parts catalogs specific to that model truck or the accessory list or the 1937 Salesman Handbook. I recall checking into this with ChevNut many years ago while working restoration of my ‘46. ChevNut served decades as Chevrolet dealer parts man and guru of Chevrolet parts on the VCCA website . I think I recall he even posted replies to others about this question? Maybe a search on the VCCA website will help? To be clear, aftermarket independent vendors did offer the overload springs for many year models. These are fairly common.