Over cooling!

The gauge being at the hot end of the normal range with a 180 t'stat is not unusual for the older temp sensors and gauges; in fact, there were service bulletins about them in the very early 60's. You may just have the wrong temp sensor; the newer ones available tend to read hotter. So don't depend on the gauge to be absolutely correct. Borrow an infrared temp gun or use a candy thermometer to check the actual temp of the coolant as it reaches normal operating temps. My '62 Dart with /6 always runs towards the upper end of the scale with a 180 F t'stat; I have calibrated the temp sensor resistance and the coolant runs truly between around 185 to 200 F in summer with the engine good n' hot; that is with the gauge getting right to the top of the normal range, and maybe a bit over worst case.

I have used the cardboard in front of the rad before with later A body /6's, when I lived in northern IN and it was consistently -10 to 10 F in the dead of winter. But cardboard was never needed in winter in Salem VA growing up, or when I went to VPI, with any of the 5 A bodies that my family owned in that area.

And as old as your rad is... 50 years!..... flushing by a profession or anyone is not likely going to get it really clean and open. Rodding it out after all these years is probably needed.... hopefully the tubes survive that. I'll be rodding out my '62 rad soon.