I was going to give BirdNest's suggestion. But next time mount the engine first. With the engine still mounted, mark the mounting holes on the firewall.

A good way to do it is to put the mount on your bench near the edge and clamp it down so it's flat. Then mark the engine mounting bolt holes. When the engine is bolted to the mount, sand the back of the mount to make it perfectly flat and then mount it to the firewall.

You would be surprised how much time you save by taking the time to get things right in the first place. It just takes some building experience to learn all this kind of stuff. Best thing you can do is meet some experienced guys in the club and find someone who's a good builder. Ask him to stop by and help you out with mounting tasks because any time you mount something to an airplane it's probably important to get it right.