Rear Mount Gas Tank for Highboy Frame?
I'm dealing with trying to mount a rear tank on my 73 F250 4x4 and I think you might be able to move the tank forward enough to clear that crossmember.
Mine is a custom made tank that I pulled from a junk yard, but I think it's close to the length of the standard tank. I think you can go up as far as where the shocks mount to the frame.