I was just brainstorming with a friend of mine and I think we have come up with a great solution. Instead of putting the tank up against the trunk floor, I will mount the crossmembers to the top of the frame rail and connect the two crossmembers above the tank with two pieces of 3/16 x 1.5 stock so the straps snug the tank up against those instead of the trunk floor. That way the whole thing is connected to the frame and it takes the frame to body movement out of the equation.

Thanks again for your help

Pat