Once I finally got the engine back, I decided to make a firewall template out of coroplast before drilling my new titanium firewall (thanks to my friend Mathieu Gratton for the idea!).
This allowed me to try different hole locations and converge on routings for hoses, controls and wires that I was happy with. Took a lot of notes directly on the template for me to remember all details as I was trying each setup.
I used the green tape on the coroplast to mark the location of the structural members of the fuselage. This helped to avoid holes or nutplates at a wrong location!
For the most critical elements like throttle and mixture cable holes, I placed the engine as close as possible to its final location (the engine leveler helped a lot for that):
After taking some time to look at all this I decided to relocate the starter solenoid as well as the voltage regulator which were both mounted on the firewall before. The battery being behind the seat, it makes more sense to have the unprotected cable between the battery and starter solenoid as short as possible (I don't have a master solenoid). Also, after looking at specs from B&C aero, it is generally recommended to locate the voltage regulator behind the firewall for cooling reasons.
Once I figured out where I wanted all holes to be drilled, I transferred the final markings to the titanium firewall and drilled all I could using the press drill. Obviously some holes were too far from the edge so had to use a hand drill for these. Most of the holes were easy to drill with cobalt bits. The larger diameter holes were more of a challenge. Biggest hole was 1.0 inch. I used a unibit but it required patience since titanium is quite hard. Clamping the Ti sheet to a thick piece of wood helped. I also used cutting oil between each step and reduced the drill rpm as the hole was gradually getting bigger. Slowly but surely, I finally got there.
Last step was to install the nutplates. I also decided to rivet reinforcing brackets at two locations where more stiffness was required: behind the 802 oil valve and behind the brake fluid reservoir. I used a C-frame to make dimples and riveting (thanks to my friend Scott!)
As a comparison, here is the old SS firewall a.k.a. swiss cheese firewall: