My 3500 crew dually has a Transfer Flow Fuel 98 gallon across the bed diesel tank in it. I put it in a little over a year ago. No problems so far but I added a Snug Top Shell this past weekend....
Here is my question / maybe problem.......I have ordered the kit from Transfer Flow Fuel that puts a thru the side of the shell fuel cap kit on it. I have plenty of room for the kit as I ordered a short bed window on that side and it provides the flat space to put the kit.

Has anyone here ever done one of these kits ? Did you have to modify the existing filller neck on the tank or does the kit have an adaptor or a big enough hose to go over the filler necks flange area ?
Just curious, my kit isn't here yet and the pic in the add from TFF doesn't really show enough. Of course they are closed right now so I can't ask them this question....Dummy me. LOL

Check out Idle_Chatter's garage. He has pictures of a similar setup.

http://www.dieselplace.com/forum/garage.php?do=view&g=190

Different brand tank. That one has a smooth neck on the tank so a rubber hose slips right on. Mine has a factory style screw on cap flange on the end of the fill pipe that I must deal with.
Thanks for the intial direction to go look to tho.