Yes I had forgot about that ,the 2 pieces of wood go on the top to help hold the FW in place .Add them after the throttle wire and tank is completed otherwise there in the way .
What I did for length was to measure the cowl length ,by setting it spinner end up on a table and measuring thru the hole.Then I subtracted 1/2"and came up with 9 inches for a reference . That gave me approx 5/8" overlap on the FW for mounting .

Just for kicks and extra strength I added some extra support to the FW It maybe dosent need it ,but I don't like the thought of it giving at all.

What prop are you going to use .I'm waiting as I type this for a 160 OS to replace the 120 YS I have in the plane now .