There is some kind of paint on the wood.
Maybe for fuel protection.Im not sure you have good bonding.
You might gluing only that paint and not the wood it self.
Go to a shop that they make aluminium windows and doors.
Where they bin there leftovers go and find some " L " profile of thin aluminium.
Cut two pieces and put them on the two sides to connect the firewall with the sides.
Use screws and safety nuts.Not only glue.
Your engine holes are on the way?
No problem ,make hole in that point on the aluminium.Your engine will hold it too.
My 2c