Temperature Question/Advice

How hot should my mostly stock 340 run? I guess a better questions is, what's the best way to tell how hot it is running?

I just had the stock radiator boiled out and resealed. It was originally a 318 car with air, and the radiator core was replaced about 10 years ago. I also put in a new stock thermostat, and flushed the system. The reason I am asking is the stock gauge (69 barracuda) is running one notch from the max. Now I know it's old and probably very unreliable. Overall everything seems really hot under the hood, but no signs of actually over heating. I drove for about 45 minutes, and it's 80 degrees and overcast...

Thanks!
Robb