Oh, the reason mine was close to overheating is I put an A14 in it, and it runs up to the H mark when ALL these occur:
* running high speed (110 kph or higher)
* running close to full throttle
* on a hot day
* up a hill

The radiator that is in there now NEEDS TO BE REPLACED to work reliably in all conditions with the A14. It's not that it needs a rod-out, or new coolant, or fins replaced. It is just too small for an engine considerably in excess of 69 HP.

But good point about fin damage or internal blockage. If that's the case, the radiator needs to come out to fix it. So repair or replacement, it needs to come out.

If the coolant is old or contaminated it should be changed. But I've never seen it solve a case where the temp runs close to the H. I've replaced coolant and done back-flush on lots of Datsuns. It certainly can help a little.

Thermostat is unlikely to cause this problem (steady middle temperature except when under heavy load).