Edge finished the bottom skin with files, emory cloth and scotchbrite. Clamped the firewall in place inserted #4 rivets in the longeron holes, and set the bottom skin on to check alignment. Set the centerline to the second row of holes as called out in the plans. This position did not exactly yield the setback called for in the drawings of aft face of firewall 5/8 back from edge. Did some research on other build sites and decided to make the forward face of the firewall 5/8 back from the edge as described in instruction manual (vs on drawings). This put the edge of bottom skin holes at the aft edge of my centerline. Sufficient edge distance. Proceeded to drill the two holes at either end of firewall bracket. Clamped the firewall several ways to get the entire front face at 5/8 inch witth #4 rivets still loose in holes. Double checked all points per plans then began drilling firewall to skin. Used fresh drill bit and Boelube on the drill for each hole, turning the drill slowly. This process gave good results in drilling the stainless. Cleco'd as I went and drilled all holes in left and right side skins to firewall.