340 Overheat / Radiator Geyser

I gotcha. Get the thing on the hwy cruising at steady 55 to 60 for a few miles. If the temp goes down, it would point towards a possible problem with the shroud or fan not being up to snuff at idle.

205-210 is normal for most newer stuff including the magnums run at. Wondering if you accidently have a 195 thermostat installed. I don't like my engines running that hot so I always put a 180 degree thermostat in my magnum engines. Even that didn't cool down my 5.9 in my van enough for my liking so I wound up installing a big capacity aluminum radiator and electric fan set up from a Dodge Intrepid. Big difference now it will cruise at 185 all day.

Good luck with the other stuff I have been in similar situations.
Thank you...The 180 that was in-there tested OK. I installed a 160 just now and started the process of a radiator flush which I'll finish Sunday. Given the history of this car (started resto in 2006 and still not driveable), I'm "certain" that this is gonna be something much more sinister.