Overheating- all of a sudden

[WinDancer]

I have been driving my truck [69 GMC w/early 68 396, double electric fans with adjustable thermostat, shrounds] for a few months now, with some work breaks in between.
It has always run very well with no problems.
The fan most always comes on after engine shut down- as it should- to deal with the temp from stopping the water circulation.
Today when I got home from work steam started blowing under the hood. I hit the manual override for the fans, but it didn't help.
The fans come on with the override.
I let the truck cool for about three hours, refilled the radiator and started it up.
I kept watch on the fans- they didn't come on. The truck got hot enough to blow some steam again, so I am parking it until the weekend.
The fans still come on with the override.
The radiator is less than a year old, 4 core.

Is there a logical answer to this or are there two problems out of the blue?

In which order and how do I narrow this down to the problem?

Engine Thermostat?

Water Pump?

Fan thermostat?

Something else?

I did a search on overheating and didn't find anything like this- running fine, now not.

How do I check each item? I don't want to throw new parts at it until I get lucky...

Thanks,
.