Made one of my multi-annual trip to the yard waste site the city maintains this morning in the pickup. Had a few difficulties, most of them indication errors I think.

The first and most concerning was the OIL light illuminated and stayed illuminated after a slow steep uphill pull in low gear. Uh-Oh! This is a first for me. I pulled over and checked. No leaks, no runs, no errors, full crankcase on the stick of good ole 10w30. New oil pump, or near enough. Everything sounds fine, closer inspection of the oil light sending unit shows it is leaking a bit. It is reasonably new, and I suspect another casualty of cheap aftermarket parts. I'll check OP with a good analog test gage to be sure. Also my electronic voltage gauge indication is markedly off at the cigar outlet versus voltage measured at the battery. I'll have to check that out more carefully due to a new voltage regulator. More import parts/tools I guess

Something else though, that has me stumped as I was looking everything over at home, I felt the upper radiator hose which was hot and pressurized, as is normal. But the lower radiator hose was cold or at least "room temperature". The top of the radiator was good and HOT, as to be expected. The lower part of the radiator was not. Never seen this before after a good run, unless maybe the thermostat is defective. Anything is possible, BUT.

Had I done anything recently with respect to the cooling system? Yes ...

Just few days ago I drained the radiator of coolant, it holds almost exactly 2 gallons. Closed petcock, installed 2 gallons of fresh 50/50 coolant and water, and felt better about the overall corrosion protection till next year. The engine itself is very clean inside, and radiator is only a couple years old. I'd ran the heater again on the trip to help ensure all the coolant in the system was thoroughly mixed which included um, a "spirited" highway run so everything should be opened up to include thermostat. Probably 10 miles or so total. 195° F. thermostat and 45° F. ambient temp

So, before I start maybe needlessly swapping in a new thermostat, is there possibly an "air pocket" or something like that going on here? Water pump failure? No leaks there, I'm skeptical of that too.

My wild guess is there was a vapor/air pocket causing the lack of proper flow through the radiator. Since you didn't over heat or boil over, there may have been sufficient flow through the by-pass and the heater core. 45* amb helped a lot to keep things cool enough. Whenever filling a system after draining, I always disconnect the heater hose from the connection on top of the manifold behind the thermostat (highest point) and fill until coolant just begins to run out of the fitting. Usually if not filled in this manner, it takes a couple heat cycles to burp all the trapped air from the system and topping it off again when completely cooled down.  Just my 2 cents..................let us know what you find.

64F100 57FAIRLANE500

Yeah, it should or might take care of itself I'm thinking. Seem to recall a "jiggle valve" on this brand of thermostat. Another method is to operate the engine at a healthy nose up attitude on tire ramps or on a hill, radiator cap removed till the thermostat opens and "burps" itself.

I've always done pretty much the same thing you mention on a complete drain and flush and never had an issue but I figured there would be no problem with just draining half the system.

Maybe the thermostat has failed open, I dunno. They sometimes weaken over time so gradually that it's difficult to notice until heat output falls off too far to ignore. I messed with the heater a little bit today mostly just to flush out the heater core with newer coolant or whatever, but it was nice enough to keep the windows down mostly so I didn't pay too much attention. Just seemed strange the upper radiator and hose was good and hot, and the lower portion of the radiator (and hose) was not.