Using a 3/4" drill bit, make the necessary holes for the gauge wiring that you've previously marked.  Next, drill the 2 remaining 3/16" fastener holes on the windshield side of the gauge mount. Only drill the gauge mount, not the factory pillar.  Then, mount the gauge mount over the factory pillar.  Line up the first 2 fastener holes you drilled (arrows) and drill through the second 2 fastener holes (windshield side) of the mount using the 3/16" bit.  Tip: you can use some screws inside the first 2 fastener holes to keep them aligned.  Be careful not to scratch the gauge mount if it was painted.