Don M. The useable firewall is 6 3/4 High X 7 3/4 wide. Motor box extenstion is size you want to make it!! Room for radial mount to firewall is one thing. Next having the engine shaft centered with mount not outside of firewall is another thing to think about. Another thing yet ...will the prop hub be in the right place with Radial mount flush with main firewall? Capt,n[] EDIT I found a photo of firewall...so added it here.