I bought a new '77 PowerWagon with full-time 4wd and the 440 in 1977 and gas was 38.9 in California then. I don't remember if it had two tanks or not, but it would pass anything but a gas station
Today I would rather not have an in-cab tank in my truck So if it were mine, I would pull that tank, plug the hole, enjoy the in-cab storage area and live with a frame mounted unit. A lot safer that way, but of course you can do what you want