Are your tank mounts welded on or held on by a 4 sheet metal bolts? Mine were held on my the bolts and I just took them off, purchased longer ones, used a 4 large stainless steel nuts, ran the bolts through the nuts which acted as spacers between the frame and tank mount, and tightened everything down. Use nuts large enough to give room for the U bolts to move forward or backward for adjustment if necessary.
If your tanks are welded on, you could still probably use large nuts or two stacked on top of each other and fastened to the mounting plate to give you enough clearance so the tanks sit flat.
Another possibility is to grind out part of the bottom flange of the tanks to give clearance. Don't know if that is legal or desirable though as it may compromise the integrity of the tank.
A third possibility is to grind/cut out a small portion of the tank mount itself - just enough to give the U bolts the necessary clearance to fit on the A frame.
A forth possiblity is shimming up the tank mount as I think you mentioned. I would cut a treated board of the necessary thickness into a shape that would fit over the U bolt and bottom on the tank to give a flat surface. Then set the tanks on top of it.
Hope this helps.
Barney
