Doug, I did not use them ORIGINALLY because of the existing triangle stock already in place. I believe the orginal prop to firewall distance was 5.2". The bad thing was getting to the needles on the carb...almost directly in the firewall. I have since moved the firewall back about an inch and now evrything clears the wall. Much simpler! I did a good job of notching, glueing, bracing, and screwing the modified box together, but I also ended up using angle stock to wrap around the outside of the firewall. I screwed the part attaching to the sides of the box, with the remainder wrapping around the front. That part was also screwed into place. That firewall will never pull out! You are right about the top standoff being a tight fit. I simply notched the original box so the base of the standoff could extend past the edge. It presented no problem.

Update...I've added pictures. You'll notice that I have used a metal plate intended to cover an open electrical box between the wooden firewall and my standoffs. (I painted it black.) It has a thin rubber material behind it against the firewall. I thought I could kill 2 birds with one stone by adding needed weight and stiffening my mounting surface with a vibration reducing subsurface at the same time. I have no idea how it will work yet...may go flying today. I did it because I came up a bit tail heavy; rolled inverted and she would climb. (dang ignition battery eliminating circuit! I love it / hate it. Love the ignition switch on transmitter, but need the battery weight to balance.)