Doesn't that car take a 22" radiator maximum? I've never had one that old with a V8, but every later-model with a V8 and 26" radiator has never suffered heating problems at idle. I use the same type 5-blade clutch fan and factory shroud on each.
You may want to look at the water pump pulley. There are different diameters that fit, so if it has a large pulley, that slows the water flow through the radiator and if it's a small pulley, it speeds it up. What happens if you rev the engine at idle to get the water flowing faster and air moving? Does it start to cool down, or at least stop climbing or does it keep going?
The fan I use is a 5-blade type from a 5th avenue or a truck combined with a thermostatic clutch. The thing about fans that's important is the PITCH, which is how deep the blades are angled and ultimately how much air they move. The clutch fans are deeply pitched, which means they move a lot of air when the clutch locks and they are noisy. Most guys disliek that but you can't have cake and eat it too. Noisy fans move air, quiet ones probably don't.
Mopar used fixed fans and flex fans also. Both have disadvantages. The fixed fan runs all the time and is noisy all the time but they move air alright. Is this a MOPAR 6-blade fixed fan? I've never seen one other than 4-blade versions. They can also do damage to fingers and to everything in close proximity if they fail from overevving.
The flex fans also are at a disadvantage because they FLEX. When you rev the engine, the pitch of the blades flattens out and guess what...stops moving air. The idea was that they would move air at idle, then they would defer to the ram effect of air at higher speeds. Mopar went straight to clutch fans on anything larger than a 318 that had AC in later years. Good luck with it.