Thanks everyone for the advice and replies. To answer these in summary,

1. To ”Mickey”, I checked out the radiator again. It does still run a little cooler toward the bottom than the top, but it should be ok as it was overhauled and retested just before Christmas. Heat naturally rises of course. Maybe 5-10 degrees is a reasonable difference top to bottom?

2. In hindsight, letting the car idle for 30 minutes probably wasn’t one of my smartest moments. Easy to forget this car is almost as old as me Yes 240 degrees was hot, but the few times I’ve driven it since then, and on much hotter summer days, the top hose temperature hasn’t exceeded about 220 degrees. The bottom hose is typically about 190 degrees immediately after the drive. So this is all due to the added air flow while driving I guess.

3. To “Bluebird 428”, “Cal H” and “Dr Drivability” thanks for the thoughts; the head gasket is fine though. The thermostat is in correctly etc,...The new sender hasn’t tripped the warning light either. I wrote to Wells about the TU46 sender. They said it activates at 263 +/- 10 degrees. This seems pretty hot to me, as I’m sure you guys would agree, so maybe I still need to look for a lower temperature sender. But I don’t have a factory spec on this, so I’m not sure.

Finally, to “Bob S” I haven’t had a chance to put a gauge in yet, but will do this in the next few weeks as a precaution. But for now I’m concluding the operating temperature is ok and that the old sender unit was tripping the warning light below the appropriate temperature effectively giving a false positive.