Based upon what I can see with your airframe, I would look for a way to mount the ignition against the inside of the firewall and route the plug wire high enough to exit the plug side of the firewall near plug height, while exiting at the inside edge of the firewall. Sorry that was so "wordy". It doesn't appear that you have enough room between the engine and the firewall to clear everything that will be there.

The positioning of the module itself is not overly critical as long as the plug wire remains clear of all the hot stuff, and it can remain relatively cool. Inside the fuselage against the firewall will meet those requirements just fine. You'll need to make an access hatch, though. It looks like you could easily do that from the bottom.