Engine running too cool

[JMD]

Hi all,
I have a MarkIV 454 in my ‘68 K10. It has a 180* thermostat, cheap 2 core radiator, and 2 temperature sending units on the back of the intake manifold. Now that the weather has cooled way off, I’ve noticed that the engine temp is dropping down to 140-150* at highway speeds. If the truck is just idling in park it will warm up to 180+ but it takes a very long time. I don’t believe it’s a sending unit issue because both gauges read within 5* of each other at all times.

I’m guessing the thermostat is stuck open, but before I go through the hassle of ripping into and replacing the thermostat I wanted a second opinion. Thoughts?