I can't imagine finding any other than a one-off custom made tank fitting in the correct location to connect to the filler door ... from your original note of the 'diagonal seam' you have the tank all 55-59 panels and burbs had, which if properly retained by the original straps nearly touching the wood floor, doesn't hang too far below the frame, unless your retainer straps are wrong .... if you plan on having the truck long, get yourself an assembly manual, fuel tanks are in section 8
Bill
