I did that with my truck. it was an 83 diesel f350 not sure if it was a cab n chassis, but it was a poly tank, I used the original straps to hold it up, and a second set to hold it down. I did use a 2" piece of square tube to form another cross member for the front of the tank, making sure to clearance the rear diff.

This is the only picture I have though, and it wasn't hooked up yet. http://i1101.photobucket.com/albums/...kfromabove.jpg

I wound up cutting the straps and shortening them, and then went back and re-bent the others to fit. I'm looking at going back in and putting in a skid plate off of a bronco to hold the tank up from the bottom.