Chris,
I did try that, but even the scribed lines look like they are off center to the firewall.
I had the cowl off mine tonight redoing the fuel lines. My mounts are centered either side of the hole with the motor pads lined up with the scribe line as I described and the spinner is centered. Maybe it is off a fraction and I corrected it when I mounted the cowl but I don't think so...
What's wrong with the stock mount? Mount one side to the motor, mount the mount to the firewall then slip the other side in place and drill the motor holes etc. It's not crap, just a bit different.