The first thing you should do is check to see if your external thermostat is working with an IR gun. The next thing you should do is check for rodent nests behind your fan with a borescope. The next thing you should do is figure out when your cooler fan kicks on (I think the stock fan controller is set to engage well over 210°F).
When do your temps become excessive? Is it when you're hard on the throttle or stuck in traffic? If it's the former, cooling will benefit from an additional oil cooler (but this would be surprising if all is working correctly). If it's the latter, cooling will benefit from a better fan. The stock fan has been monstrously outclassed by some $50 modern fans.
Changing gauges will not increase accuracy—they all come marked with temps. 180°F is a bit below the first tick. 210°F is a bit above the first tick.
Before you install that AC stuff...Griffith makes a system that is far superior to the factory setup. Classic Retrofit just came out with an electric cooler that (by anecdotal accounts) is somewhere between stock and Griffth in terms of performance, but moves all weight to the front of the car where you want it. Both are $3-5k installed.