I've got the fuel tank installed and fuel and vent lines are bent and installed!
I cut out the rear crossmember and trimmed the inner frame rails approximately 3/4" on each side and some light trimming on the lower rails for the tank to fit.
I got some 1/2" rail and 3/4" angle to get the tank level. I drilled and tapped 5/16-18 holes to bolt the tank to the mounting frame and ran bolts all the way through to the truck frame to secure it.
This shows one of the tapped holes.
Here is what I had to grind off to level the tank in the front with the hump in the frame.