Rich:

Lay a large piece of paper on a flat surface and draw horizontal and vertical centerlines on the paper. Set the back of the cowl on the paper and use a plumb-bob or a square-ended ruler through the prop-shaft hole to center the hole over the junction of the centerlines on the paper. Without moving the cowl, trace the outline of the back of the cowl on the paper.

This will tell you where the prop-shaft will exit the cowl in relation to the firewall. If you have no right or down-thrust, just measure the distance from the top of the cowl to the centerlines on the paper and use that measurement to find the distance from the top of the front former to the center of your engine mount.

If you have right thrust, offset the engine mount 3/16" to 1/4" to the right (looking at the front of the firewall) and if you have down thrust (unlikely in an Extra), offset the engine mount upward 3/16" to 1/4".

After you have made all of the measurement, glue the engine mount to the firewall and check that the prop-shaft will come out where it should before you drill the mounting holes.

Jim