Overheating during idle
That is what I thought I might hear. After doing the things I listed it is now idling at about 210 and if I am correct coolant boils at a higher temp. Do you know the volume of diluted coolant in a 340?
Coolant will only raise the boiling point 11 degrees. Don't need it in So. Cal.
That is not the core of your problem. Good chance you'll have to drain the liquid in the system to find/repair the core problem.
So it gets to 210 degrees, starts boiling over, and you turn it off? Have you actually driven the car since you got it running again?
What did the water look like the first time you drained it or flushed it? Have you drained the block itself at the drain plug in the block? Have you stuck a coat hanger wire in there to check for sludge rust? What did it look like when you removed the water pump.
Some probabable causes for overheating at idle.
Timing
carb tune
ignition tune (plugs, wires, points, advance)
fan shroud?
even though you flushed the system, there might be rust blockage and scale in the block
Do you have a laser heat gun? You can get them at Harbor Freight for like $35Check it at the intake manifold before and after your thermostat. Check the temp and various place all over the fins of the radiator. There could be tubes . You need one.
Check the temperature of the water at the top vs bottom radiator hose. that are blocked.