With the assistance of my friend Jim, I cut a 2" hole in the firewall for the cabin heat box. (We removed the engine mount to get better access.) I used a fly-cutter to cut the hole, which worked fine, but was very slow. I had to remove and sharpen the cutter at least 10 times -- the stainless steel really dulled it quickly. Jim held a 2x4 on the back side while I drilled. I dressed the edge of the hole with a stone wheel in a Dremel tool.

I have a stainless steel cabin heat valve from EPM.AV Corp (not stock, of course) which is great, except it requires some changes to the stock installation. In my case, the firewall-forward part of the valve will be mounted in accordance with Drawing OP-34, but the aft portion will be turned 90 degrees so it misses the nearby firewall angle, and leaves room for the control cables to pass through the firewall. This new attachment method means there will be four bolts to attach the box instead of two. No biggie.

I had time for one additional task: I drilled the bolt holes that attach the instrument sub-panels to the side longerons, one on each side.