The only issue I can think of with mounting it on the firewall is to be sure there is adequate cooling. Cooking the ignition is not a good thing. Cubs don't usually have large firewalls, you may need to consider mounting the ignition behind the firewall with an opening to allow the plug cord to come through to the spark plug. I did that type installation on a Chipmunk, and it was a happy solution. I provided foam padding for it to minimize vibration.