I'm with Tom, check the inside of the firewall or sand the front face off and look for the plugs. If you need a little extra spacer use the plugs to determine the location for the box on a new piece of plywood and then use the current holes and blind nuts to install the secondary firewall with the box on it. The old holes will be put to use and it will look like that was the way you intended to build it. Yu also get the added bonus of my weight up front to balance the airplane out.