I am sorry I cannot help you much then. I am pretty new to all of this myself.

Just a thought, and I would defer to someone with a great deal more experience thanI have, but I wonder if you could epoxy the 1/8 ply to the front of the existing firewall, and then use several small screws to help hold it in place to compensate for the weaker epoxy bond between the fuel proofed wood and the new ply. I would think that with the bolts for the motor mountsandwiching it all together that thiswould be pretty strong. Again (Disclaimer) I am pretty new to this so I would default to more experienced flyers.