I went to buy a car with my friend last night and while talking with the seller he mentioned that he had a 65 c10 in high school. While we were talking I mentioned that I moved my gas tank to the rear. It must have triggered something because he mentioned to me that a 75-79 F250 gas tank would fit right between the frame rails without mods and the tank would only sit about 2-3 inches below the frame. He mentioned that it was about 18 gallons. Just thought I would share and see if anyone has done this.