Yes I searched before posting. My 340 is a "fresh" motor but broken-in. The car has been plagued with issues so I'm just getting to where I can drive it more than just around the block. I've taken it out maybe 3 or 4 times and it's always been running real hot. 3/4 on the gauge or more. Tried some more defined troubleshooting today. After the last overheat last week, I topped-off the radiator. Today I cranked it up and coolant immediately discharged from the overflow hose. OK it was probably overfilled. After a while it stopped, but the temp quickly rose to 3/4 and hung right around there. Heat is hot. The radiator cap was OFF from this point on. There's some bubbling in the radiator but the level remained consistent. Another thing I noticed was it seemed like it wasn't circulating. There were a few specks of orange engine paint in the coolant that stayed right there at the opening the entire time. I let the car idle for 35 minutes and there was really no change. The gauge would creep up to just over 3/4 at times, and when increasing RPM's the temp dropped a little. HERE'S WHERE IT GETS GOOD...As soon as I shut it down, the radiator exploded like Old Faithful from the filler opening. Probably a 2-foot geyser. So bad that it would have been catastrophic if any of you goons trying to help had been peering down into the radiator. It blew out a lot of coolant, and then I could hear boiling from the motor.
This is a stock 340 rebuild. Recored radiator. Oil looks good. No steam from tailpipes. Has a fan clutch but the fan spins all the time. As I said I've had a real hard time getting this car to where it can be driven, and it's been a long time but I really think that this problem is not a "new" one, rather it has been this way since the rebuild.
I have no confidence that it's something like it being airbound, but only because I've had small block Mopars for 40 years and never had to deal with any of that. Thanks, guys !!
I do have IR thermometer data and video of the bubbling and the suspected lack of circulation if anyone wants to see it.