Just have someone weld a bead down two sides of the third leg if you don't want to drill the holes.
Another idea: Not sure of the spacing but could you fit something like a flat piece of 1/4" steel between the rail and the gas tank? If so then drill away as by time you drill through the frame and hit the plate you will know to stop.
_________________________
FSRBIKER
97 TJ, 2" lift, 32's
69 Bronco, locked Dana 44, 35" MTR's, 5.5" lift
EOE...Opening Soon!