Overheating at idle
if it were me...
put a stock water pump and thermostat in your cooling system. A stock mopar water pump flows 20% more than a GM or Ford. The stock thermostat will keep your coolant in the rad longer.
Get rid of the clutch fan...they operate at about 80% speed at idle due to fluid slip.
Try a little more initial timing. Retarded will put the fire in the heads rather than the cylinders where the coolant will absorb the heat. A good test is to check and see if your headers glow in the dark.
If that all fails..just open the vault and get a griffin muscle car rad with 1 1/4" tubes. Then go find a red light and sit there without worry at an ambient temperature of 90 plus.
I was told in the early 1970's that a 340 A/C WP impeller had one less "blade" on it.
I believe Caterpillar rads had alternate tube rows at a slight angle so that the air exiting from between two tubes in the first row, was directed at the sides of tubes in the next row.
2 row, 3 row, and 4 row radiators are somewhat counter productive. All the coolant is passing through all the tubes at the same time. The front row gets the coolest air. Each consecutively gets warmer air, reducing its effectiveness.
A cross counterflow radiator is more effective. You can not buy them, you have to build it. Your coolant will flow horizontally like most new vehicles. You look for an aluminium radiator with a width that will fit your core support. Take a measuring tape to a wrecking yard to find a couple of vehicles with adequately large cores. Then check for new radiators that are less expensive from the same manufacturer, you will need two.
With the plastic tanks removed, you need to build or have a welding shop weld hot and cold tanks with hose connections. These will both be on the left or right side, so the tanks will be tapered so the one hose will pass the other tank. For the other ends, you need a small radius 180° aluminium bend to weld between the two tanks where the coolant will make the 180° turn out of the rear core and into the front core. Then you need another piece for the outside and two end caps to seal it up. On the hose connection end 3- 1" tabs can be welded between the welds between the tanks and cores. Small "channels" can be bent and welded to the left and right ends to seal the air between the two cores. A bit of foam pushed between the tabs to seal the hose connection end tanks.
Hot coolant goes into the top hose connection on the rear core and the coolant returns to the WP from the lower hose connection on the front core. The hottest coolant is cooled by the warmer air. Then the coolant returns to the other end through the front core to be cooled by ambient air. This should keep coolant flow velocity in the tubes high enough to be turbulent, increasing efficiency.
Paul Lamar described this in his royaryeng.net blog or chat. Yes I know, rotary engines and aircraft. Paul has diagrams for this in the discussion. Consider this, rotary engines are notoriously difficult to cool, aircraft run at 75% power for hours, and weight is a concern. Paul passed about 4 years ago. Sad loss as he was very intelligent and inventive. Should still be able to find the site.
I just tried to find the original site, but since Paul passed, it has been changed. I sent an email to see if the original site can be accessed or if I can get a pdf of the cross counterflow radiator process. Paul Sr used two Dodge diesel radiator cores to build a radiator for a time to climb aircraft. This light kit plane had a 650HP two rotor engine that ran on methanol. It is now at the Smithsonian.
They had problems with the injectors getting plugged with corrosion in the aluminium fuel lines after sitting for a week or two. I wrote Paul and explained that Indycar teams drained the methanol every night and put a gallon or two of gasoline in the tank and started it until it ran crappy, indicating the fuel system was purged of methanol. The next time they needed to run the engine they drained the gasoline and put methanol in.
They started doing that with the time to climb engine, and had no more problems.
Hopefully I get a positive response from Paul Jr or another admin.