Are you sure your timing is right?

Running at the 'M' is normal if you're running a 195 'stat; 'T' if you're running a 180. If you're hitting the 'P' then one begins to suspect the radiator. (this motor runs fine with a 195deg thermostat, electric fans and everything, very tough to overheat... generally overheating problems end up being a radiator that needs rodding out.)

The phenolic spacers are indeed to slow down the heat transfer from the manifold to the carb. They do not affect your cold starting. And you don't mention a hot start problem or vapor lock or anything, so I would set those as a low priority. You seem to be doing fine without.

Your mixture certainly isn't excessively rich (for the reasons Al mentions, and from the color) but it may be a little on the lean side. A quick check involves lifting the piston manually while the engine idles warm -- John Twist of University Motors gives fantastic explanations of how to check this on YouTube.

The quickest way to check that your choke is working: pull it out when the engine is idling hot. You should notice a big difference: It should promptly start running badly. If it runs perfectly with the choke pulled full out on a hot engine, the choke ain't workin' and the mixture ain't rich enough for cold starts.