I attached the newly cut ply firewall to the original firewall- or what was left of it. The outer original firewall was still intact- the motor went thru the center of the cheap Chinese ply, so I just cleaned up the cavity, added a few braces, and screwed the new firewall to the old firewall perimeter. Yes- if u reinforce the thin firewall, and add ur new ply firewall, set the cowl on front, and measure distance to the prop hole. When u decide what motor mount u will use, bolt the motor to the mount, and set it up against the new firewall, and see where the prop hub comes out to the cowl hole. When I did that, I had to add another small ply plate behind my motor mount. The type of motor mount will dictate where ur motor will fit on the mount, and where the prop hub protrudes through the cowl hole. My mount wasn't real long, that's why i had to add another block. Just do a trial fit of everything before u commit to drilling any necessary holes. Dean