My 78 D300 had the in cab tank, since it was a cab & chassis. The regular frame tank wouldn't work on these trucks, one needed the left side of the frame open for a trans mounted pto (to power the hoist) and the other side for exhaust.

I used a rear tank from a 77-78 club cab shortbed truck when I restored mine. The donor truck was dual tanks, one in the standard frame location, the 2nd one in the rear where the spare tire would be.

Ed