72 Slant Six running too cold

Agreed on that cap. I think now it's a waste of money.

I'm certain that the engine is getting to at least 180, since it's pressurizing the system, and that upper hose is hot. I installed the new thermostat the same as the one that was in there...pointy-side out.

The more I think about this, the more I think the gauge, or more likely the sending unit, is faulty. Yesterday it was 60, and I put a piece of cardboard in front of the AC condenser. It sucked it up against it when it was running, and it covered most of the surface area, yet little change in the gauge.
Can you hook an aftermarket electrical temp gauge to that same sending unit, or does it take a different one? I know if you go from an idiot light for the oil pressure to a gauge, you have to change the senders, but not sure on the temp. I may grab a cheap one and just hook it up under the hoot to see what it reads, as that sending unit is plumbed right next to the thermostat in the head.

Of course the heat was still not super hot.

I'm pretty sure the blend doors & heater core aren't the issue with the heat. I just replaced the heater core a couple of days ago. I made sure it was all cleaned up in and around the heater box. The car only has 55k miles, and while I was under the dash, all of the vacuum lines look nice and pliable. When I switch from heat, to AC, to Defrost, everything blows from where it's supposed to. It's warm, and definitely better than it was (the heater core was bypassed before), but definitely not super hot.

Only other thing is I'm fairly certain the radiator was replaced in the past couple of years. Maybe they got one for a V8? Of course if it;s getting to operating temp, it shouldn't matter, as that's what the thermostat is supposed to do.

OK, I'm starting to overthink this....