Hmm! You could mount the cowl then eye ball the center where the prop shaft goes. Maybe even tape some string on the opening to mark the center. But if the firewall has right thrust and down thrust built in you have to account for that. All is not lost if your off on your first attempts. Just plug the screw holes with a wood dowel and expoxy then sand smooth and retry.
I always mark the firewall engine center prior to gluing. Just did all this a few days ago on a P-38.
Edwin