Those with fuel cells under bed, I need help with mounting it.

[REPO1]

I have the Summit Racing fuel cell that I want to mount between the rails at the rear underside of the box. The question I have is regarding how to mount it. It came with 2 U straps that go around it. Now what I need to know is if I need to mount it so that it sits flush with the top of the frame, or like I seen in a picture, sitting slightly higher than the frame.

But the problem I can see coming is with the box crossmember that goes across right above it. I think it might hit the fuel filler neck that is stiking up right there.

Any pics, advise, help appreciated!