1. I cutout a 3/32 plywood ring exatly the same size/shape as the nose of the plane.
2. Find the dead center and drill a hole the size of the motor shaft.
3. Then CA tack it in about 8 spots around the diameter of the nose on the outside. CA tack the spinner backplate to the plywood ring. (Dont worry it will come off.. just use little dots of CA all the way around about 1/2" apart)
4. Lay the motor mount in the engine compartment and the slide the motor into place through the hole you drilled in tho plywood.
5. Put the engine through thespinner backplate, a dummy prop and install the prop nut. The motor is now set in place for the perfect spinner backplate clearance in the dead center.
5. Now bolt the motor to the motor mount and mark the location of the motor mount and mounting holes on the firewall.
6. This will allow you to
a. set the firewall in the perfect location.
b. shim behind the motor mount to make it fit correctly against the firewall.
7. Once the motor mount location is set and marked and the firewall is in place, pull the engine and motor mount, remove the plywood spacer.
8. Install the motor mount!

In the pic of the spacer, I just had the bolts in it for a test fit at the time I took the picture. Obviously the bolts dont go in until its all set in place.