The time you're running the engine at high ambient (outside) temp isn't anywhere near as important as what temp your engine is seeing while you're running it.
First: DON'T TRUST YOUR ELECTRONIC TEMP GAUGE It's about as accurate as "Fox News" Get a dip-stick thermometer from "Mainely Custom by Design" Best $50 bucks you'll ever spend. It will tell you your true engine oil temperature. Now.....Some people on here will tell you that your cylinder head temperature (CHT) is more important to know, and that's fine, but it is somewhat more involved to install a CHT system and, for your question about running your A/C on a hot day, engine oil temp is just as informative.
Second: Keep your revs up! Nothing under 2,500 rpm and I wouldn't be afraid to run it at 3 grand minimum around town in third. Why? Because it is AIR COOLED and higher revs means higher revs on the fan, too, and THAT is what will keep your engine cool(er) and alive.
Third: Once you get that dipstick thermometer, don't ignore it! Also buy a pair of hd leather gloves and check the thermometer in various driving conditions - after a while you'll get a feel for what temp it should be running under different conditions. If you're good, you can also sort-of calibrate your semi-useless dash gauge with the thermometer and the dash gauge will begin to be almost useful.
Fourth: The all-important temp ranges you should be seeing on your engine (in Fahrenheit). These are my guidelines only, and some on here may dispute them, but here you go:
180 - 210 = absolutely gorgeous
210 - 220 = Getting pretty warm
220 - 230 = Getting really hot
above 230 = pull off the road until it cools off.
Hope this helps......
Gordon
The Speedstah Guy from Massachusetts