RE: Nosen P-51 (not). MY OWN GIANT PLAN BUILD
I am going with a new ply firewall on both side of the existing firewall to give me a smooth surface on the back as well,, this way I don't have to work around the existing hole when I install the four 1/4"-20 blind tee nuts on the back, that the 1/4" aluminum motor mount well screw into.
I have the back firewall ply being held tight with the screws which I well remove when the epoxy sets, than glue in the new front firewall ply.. after the front 1/4" ply sets I tack on wood spacers and drill a 2" hole to clear the starter spring.. I am not worried about the additional weight, it just means I can take out some of the lead ballast from the front.