These were a bit more complicated because there is no way to hold them completely steady when drilling. I did as Jon Croke suggests and used a 1/8 cleco at the front. Drew center lines on the mount, placed it in place and looked through the hole in the front of the firewall to sse that it was lined up and then taped the mount to the floor and marked the front hole with a sharpie. Removed the mount and drilled the hole and clecoed. Then for the 3 holes on the bottom, I laid a weight on the mount and gently pushed the drill up from below to mark the spot...couldn't put much pressure even with the weight on it but it was enough to mark the spot. Then I removed the mount and drilled the 3/16 hole and enlarged the #20 hole in the floor to 3/16. Then once again I used a loong reach cleco to hold the mount in place and drilled the other 2 holes to 3/16.
Also installed the passenger and pilot seat supports after drilling large holes for grommets for future wire and/or fuel lines. Riveted those from underneath.
