I just suggested the 3-gauge one because all the stock gauges are notorious for not being accurate, so if you're going to go through the trouble of routing lines through the firewall and mounting something on your dash it may be worth it to do everything at once, this way you know you have good accurate readings when you're driving around. But you can always just buy a temp gauge by itself and install it. Like others have said, you can really only trust the stock ones for relative changes.
If you just want to test the temp at idle and you don't want to mount something on your dash, then use one of the temp guns that was suggested. Or another option is to just unscrew the stock temp sensor in the engine, buy a mechanical temp gauge, and screw it in to where the stock one was. Run it, check your temps, and then plug the old one back in when you're done.
1966 Mustang Convertible. 200 I6 / C4 with large log head, Autolite 2100, COMP Cams 260H, Duraspark II with MSD 6a, and dual exhaust.