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this is a good question and I'd love to see an answer...
I saw a '77 truck today and it had a side saddle tank, driver's side, with the filler neck at the front of the left rear wheel well. I did not ask the guy if it was stock. If this is the tank you speak of, I want one. I wonder what years they came on so I can go scour some boneyards...
I have a '78 Supercab<! I am parting out> and it has a long tank under the cab, just inside the frame rail. If there was room (front to back) I think it could be made to fit under the '70. The aux tank is over the rear axle and because of the wider frames of the later trucks, I don't think it will fit. However, I'd like to try as I want to put both tanks between the rails of my '71 (has cab, plus 2 saddle tanks) in a similar location as the '78 had.