If the firewall has been fuel proofed you may have a problem......as the wood has already been epoxied you won't get good penetration from the new epxoy........if that's the case it would be best to replace the firewall......
If it hasn't been fuel proofed then the 1/8 ply would probably be ok....
You ought to call the people you bought it from and tell them they can have their plane back...or reduce the price...of course they don't have to do either....
good luck on your fix</p>