If the filler neck clamps & filler neck rubber hose are properly secured and in good-order, you will not have any fuel fumes in the cab. Have you corrected any issues you may have with these items or checked to see if the tank is leaking anywhere? I have corrected these issues on may factory in-cab fuel tanks over the years and the results were a fume-free cab, without the extra cost of relocating the tank, to the rear frame rails.
