Found my dads original 1971 duster!

I wouldnt run that gauge, you dont get enough of a detailed read of the temperature in the lower (<200) range. This is what I currently run, it works fantastic, is easy to install, and comes with different adapter fittings for the sending unit Autometer Temp Gauge Autometers arent cheap, but you get what you pay for, and when you are monitoring vital engine stats, you want accurate and reliable gauges. Of course all this is assuming that you have no problem with mounting a non factory gauge in your car. That link is to the Autometer US site, so its USD, but you can find it on Amazon.ca without too much problem. You would also need the black "cup" holder or a single mounting panel need to mount the gauge to a solid surface, as shown Here
And you are correct to wonder if your temp gun is accurate, or if you are reading wrong temps, all this is part of figuring out what is going on. And thats why I think you need to get a consistent gauge reading from a reliable sending unit. Solve one problem and move onto the next. Even if you put a good gauge in, and your temps are still higher than normal, at least you have eliminated one potential point of error and you can focus on something else.