How hot is hot?

In the '80's I had built a 383 A-body that would behave similarly, although with engine-driven fan and a 3-core 22" radiator. Temps would sneak up especially on hot D.C. area summer days.

As a test I got the seven-bladed steel fan and a/c water pump pulley (which turns the pump faster) and mounted it rigidly without the viscous coupling. It made a lot of noise and wasted a few hp, but the temp then stayed at 180 like it was nailed there. That told me for sure I did not have enough cooling system. It's been a long time but I think I ended up using a flex fan which was quieter and still kept the temp right on 180.

Keep working it, would be interested to know what fix you come up with.