Once the thottle pushrod was installed I epoxied the firewall assembly on. Then I built the rest of the plane per instructions.
I did have to cut a little into the bottom for the muffler but filled that in with a balsa block hollowed out for the exhaust. I hated cutting the cowl for the muffler and other engine parts, but I could not find a Pitts style muffler for a .10! I also cut out a little on the bottom for air exit, and blocked off the intake hole opposite of the engine head.