Nice motor, just put one on a Christen Eagle.

You should have 2 lines on the front of the firewall, 1 horizontal 1 vertical

The horizontal line you want to be the crankshaft centerline.

The vertical line will also be the centerline of the engine, measure the distance between the engine mount holes and divide by 2, use this distance to measure from both sides of the verticle line and drill your mount holes. The vertical line will be offset to allow for right thrust.

I used 8-32 socket head cap screws and blind nuts.