I have almost the same truck, a '75, and my advise is to do what I'm about to do... Drop the underbed tank(s) and put in a bed-mounted fuel cell. The under-bed saddle tanks are unsafe anyway, they're too susceptible to damage/or exploding in an accident, being that they're mounted outside the frame. These trucks were recalled a while back because of this if you dont remember, but even then, poor design severely limited what they could do to remedy the situation...a plastic skid plate of some kind I was told, ooh, impressive! ha ha.
Anyways, good luck.
jason