The best place to put the gas tank is behind the axle, under the bed, between the frame rails. 16gal fuel cells work, as well as the mid-late 70s rear aux truck tank or have one custom made, they are really not that expensive if you check around.

Batteries go anyplace you fit them in. Under the seat is really not a bad place. On my 50, I mounted the battery behind a door behind the running board under the cab that would have worked out well IF I hadn't went with a snow plow on the truck and needed a 2nd battery. I suspect behind the running board, under the bed, in front of the rear tire would work as well, pretty much wasted space there too. Gene