Engine overheating

Yes you can till do the test without the thermostat being in there.But what would be the point?; you already told us it doesn't "overheat" with the stat out.

You already drove it 310 miles without a boil-over, so again it ain't overheating.

Here's the deal; the engine ain't TOO hot until it boils the water, which happens at about 100*C, with the cap off. You don't need an IR gun until it boils the water cuz water always boils at or near 100*C at atmospheric pressure. But you can't measure anything over 100 (or under for that matter),without the IR gun, which are cheap to buy these days.
If it boils the OIL, then it's near-death too hot, and this can begin at or near 120*C,so don't go there.
Modern engines run at around 110*C.
Our old engines like 90 to 95.

Now in post #20 you say you have 2 dash gauges running high. Well those two just happen to have a common voltage regulator, called an IVR. And so, the problem is pointing directly to that unit. Replace it, and start over. But before you do, check your charging system voltage output. If it is too high, it will drive the IVR high also, and then it causes these two gauges to read higher than "normal".
I've been using those two gauges as charging system voltage indicators for years and years and years. If one rises or falls from "normal" then I immediately check the other. If it's doing the same then I know I have a charging system issue.