To fill the old holes in the firewall find wooden dowel that will fit into the hole. A snug fit is best. Use a little bit of epoxy in the hole and then put the dowel into the hole. Then use a razor saw to cut the dowel off flush with the firewall.

For setting a blind nut. There are several really good "tricks" that can be used. Here's what I do. Get a wire pushrod and slide it into the fuselage through the hole you want the blind nut in. When it's in the fuselage slide your blind nut onto the pushrod and then use an easy connector behind the blind nut. Now pull the blind nut back through the fuselage to the backside of the firewall. Once it's up to the wood pull it into the wood enough to set the teeth into the wood. Remove the pushrod and then thread a bolt through the firewall into the blind nut and tighten it to completely set the blind nut into the wood.

Ken