Decided to mount ECU box on left side of the back of the firewall. Mainly because this appears to be the best place based on main calbe routing. Drilled 2" hole in firewall and installed cable thru hole and then used the provided split grommet and sheild to close off firewall. The other ECU cable will mostly be wired behind the firewall so no special hole needs to be provided. The Engine Monitoring cable will be roued thru a hole on the right side of the frewall where the MAP sensor and shunt will also be installed. Need to order electrical items from Aircraft Spruce so I can continue electrical installation.