I would think the instructions would specify how far it should be from the firewall to back of the spinner backplate. You usually put the spinner backplate on the engine and the engine on the installed motor mount and take a measurement to set the engine at the right spot in the mount according to the instructions,,, and then mark it to drill the holes. Sounds like you mounted the engine too far out on the mount to me. A 120 should fit under the cowl and line up just fine...