A couple ideas that may be worth exploring.
17* at 800 rpm should be OK. If that's distributor controlled, be sure that it stays at 17* below 800 rpm.
You could try a tad richer at at idle. pre-smog era 13.2:1 would not be unusual.
What I was going to suggest as an experiment was adding advance when the coolant temperatures go above 220 or so. That's what was done on some late smog era vehicles. For example my stock '85 AMC idles at 12* BTC, 650 rpm but when a temperature control switch (HD-CTO) sees the coolant is 220 F its lets manifold vavuum to the distributor advance.

One point of the above experiment would be to see if the source of the problem is excessive heat being put into the block by late timing.

My guess is more likely the heat isn't being carried out of the coolant at idle. So the problem is either the radiator to water or air to radiator heat exchange.

I know from the shop manuals and the parts books there were different diameter and different pitch fans. IIRC there's and 18 and an 18.5" diameter. Need the one that fits tightests to the fan shroud. The pre-70 fan blades were notched to clear the bubble top tanks. I would like to think that the repop radiator has similar heat exchange characteristics to an original - but I don't know that's true.

My guess is its air flow issue or heat exhange with the air, but would keep all the other possibilities open until proven otherwise. really hard to test directly - would need a data logger and two thermistors to see if the air is heating up or not as it goes through the radiator at idle vs. on the road. But if access to such things, then that
's what I'd do. Actually even one air probe would work - you can assume the outside air temp.

Pictured below is an 18" for use clutched drive