I don't know what size your plane is but the most positive way is as follows. If your engine is on motor mounts, mount it. If it requires right thrust, ca your spacers under the left motor mount base. Now Hang the plane vertical, place the engine on the firewall and put the cowl over the engine. Locate the cowl and tape it in position. locate the engine to center, remove the cowl and mark the mounting holes. WAALAA perfect location. If mounting directly to the firewall ca the spacers to the back of the engine and do the same. If its a big plane hang it outside from a tree if you have to, I do this all of the time with large planes. Ken....The "BALSA BANDIT"