Unfortunately the frame differneces between the 3100 and the 3600 result in everyone's experience with 3100's not being relevant to my 3600 efforts.
Tnaks Inc. appears to have some options that may work that I continue to explore and will provide information for the good of the group as I make progress in the effort.
I decided on a tank from Tanks Inc (see my project journal for more information).
It fit into the frame without modificantions to either the rear frame cross member or the angle cross bracket near the rear of the differential.
The fill will be on the passenger side, through the rear fender but I have not decided on the exact type of filler yet. Several members have provided options they have used that I will be reviewing.
The tank is narrow enough that I should be able to run dual exhaust out the back without any significant problems.
I measured one of my old bed mounting cross rails and it appears to be approx. 1 7/8" high so the filler pipe will need to be 1 3/4" max, probably more like 1 1/2" to have clearance to the bottom of the floor boards. The Tanks Inc 16 or 18 gallon tanks (noted above) have a 1 1/2" ID filler hose so that should be okay if I can get a short enough offset to get the pipe from below the frame top flange to above the top flange.
Your cross sills are indeed 1-7/8 tall but remember they sit on 2"+ wood blocks making the total space between your wood floor and the frame about 4" in the area where you propose to install the tank. That should make it relatively simple to make the connection over the frame if the tank inlet is near the top of the tank.
59 fleet, thanks for the confirmation of the space available. Look at the pictures in my project journal to see how the tank is positioned and the filler-to-frame clearance.
Moileortho, I am also interested in your filler tube routing and any additional information that you can provide regarding the Jag fuel filler you used.
Off topic a bit but I would also appreciate more pictures of your exhaust routing over the rear axle and past the fuel tank as I am going to have similar space constraints.
