First off I want to thank everyone for the help and suggestions.
I had thought about finding the original hole locations by looking on the back side of the firewall, unfortunately that didn't turn out to be as easy as it sounded. The previous owner drilled the original holes oversize and used a 1/2" dowel to fill the holes which also removed all the indentations from the blind nuts.
I think I can take some measurements from the cowl and find the center lines for the prop shaft to transfer to the firewall.