Cabin heat in most airplanes comes from a heat shroud on the exhaust system, which warms up the air before passing it through a heater valve and then to the cabin side of the firewall. The heater valve that Van's sells is made of aluminum, which will melt at a much lower temperature than the stainless steel firewall. I decided to spring for the replacement stainless steel heater valve made by EPM instead – I bought mine through Avery for $99. That's $34 more than Van's part, which I figure is worth it to not worry about the potential of having a big hole in the firewall that doesn't keep fire out.
I used one of my fancy hole cutters to put a 2" diameter hole in the firewall where the heated air will pass through into the cabin:
The steel heater valve comes with a small tube of Biotherm 100 sealant, which is pretty neat stuff according to the datasheet. I applied a gasket of Biotherm to keep fumes out of the cabin.
Here's the valve bolted in place, viewed from the engine side of the firewall:
For posterity, here's the cabin side too. Since it's bolted straight to the firewall without any extra stiffeners, I used wide AN970 washers on the back to spread out the load. Probably overkill but that's how I roll.
Since Biotherm seems like such useful stuff, I ordered a whole tube of it from Diamond Tool. There'll be plenty of places to use it on the firewall to keep bad stuff out.