The distance from the prop flange to the top of the firewall is 37".   The top of the firewall is bent out in this area so the distance to the mount bolt will be slightly longer.  The mount has a 3deg cant to the right, so measuring from the sides gives me different numbers, but here they are for your entertainment.  On a horizontal line measuring 10" out from the center, both on the prop hub and the horizontal line of rivets on the firewall between  the top mounting bolts.  I get 37 7/8" on the starboard side, and j38 3/8" on the port side.

Hope this helps

Bob Perkinson

RV-9  13B