I finally broke down and decided to go ahead and rivet the firewall recess in place. It has been very nice being able to reach through the hole to work on the firewall. For now at least, when the top skin is off, I can still reach both sides of the firewall. Once that skin is in place any riveting on the firewall is a two-person job. The rivet tape is covering the joints I filled with biotherm to prevent gas leaks into the cabin. I used the tape to create a smaooth finish. Didn't work all that well, but it came out just fine. I will back up the corners on the box with some stailess tape and a second round of biotherm.
Since I first fitted my engine mount I have had clearance problem with one of the nutplates that is used to attach the fuel pump to the firewall. The nutplate is placed per plans. If I had known about this back when I build the firewall I would have moved the fuel pump up 1/2". Ohh well, I spoke with Vans and decided to just remove the nutplate and flip the bolt around. The is just enough clearance between the mount and the bolt head.
