Not sure if this will help. I had to switch out tanks in my dodges. I put the tank from my 69D 100 into my 70D 200. My tank sizes were different but I was able to make the tank from my 69 work in the 70. The lower bolt holes in the tank flange lined up with the tank mounting holes in the 70 but since the tank from the 69 was smaller the top tank flage bolt holes don't make it to the upper monuting holes on the truck. I had to use a couple pieces of scap metal to fabricate plates that I could bolt to the tank flange and that would then reach the mounting holes on the truck. That was no big deal. But then that changes the angle of the filler neck hole in the truck to the hole in the tank. (I "assume" you are talking about a tank in the cab behind the seat) Any way I purchased new filler neck rubber seals and with a little soapy water on the rubber of the seal was abel to make the seal give enough to get the filler neck into the tank. However if I fill the tank completely full I got a little gas smell in the cab. So I got ahold of a plastic fuel tank from an 85 Ramcharger and mounted me a new tank between the frame rails under the bed of the truck.