my '91 Phorde ranger supercab had it's tank between the frame rails next to the drive line. It was skinny and long and plastic, and held 22 gallons if I remember correctly. You ought to be able to get two of those cheap and put one on each side of the driveline and get lots of tankage. The other tank that should fit easily between the rear frame rails is a Jeep CJ-7 or Wrangler tank. They are pretty narrow and fit right behind the bumper. You can get them in oversize capacities for cheap from a 4WD parts supplier. They have all the fittings for modern return-line systems too.

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