[Mootie]

Well Brian my 1970 had pretty much the same symptoms--I didn't get quite that hot but same scenario-around town or idling even in 95 degree heat no worries-take it for a 75mph blast down the Interstate and the temp guage would creep down
(always liked that engineering) to the point where I would be watching it instead of enjoying the ride. Anyway-problem solved big time-it wasn't cheap and I did a bunch of stuff at once meaning not the scientific approach but heres what I did.
--Bought a kick-ass Rodney Red aluminum radiator
--Milodon aluminum water pump
--found an NOS fan shroud (mine was a bit buggered)
--new Flex-a-Lite HD Thermal fan clutch
--Complete March underdrive pulley set
--Used some Home Depot type foam pipe insulation to seal any gaps around the shroud

Like I said--turned out to be expensive and a LOT of parts--it was one of those projects that took on a life of its own after I ripped into it.

Results were great though, probably the biggest difference maker was the Rodney Red (though I can't be sure as I did all this pretty much over a weekend). Jerry makes an awesome radiator, the weld quality and finish is great-no fitment problems at all--the pulley situation evolved out of my installing the new water pump(correct for my year/engine) and as I was zipping it up I noticed my PS and Alt pulleys were out of line at least a 1/2 inch from previous. From the boards I learned that the cast iron pump on my Judge was the model that 1/2 inch shorter than it should have been--but the best part is the previous owner corrected the misalignment by coming up with his own concoction of brackets and pulleys! They were chromed, sure, I just thought they were what should have been on there. The guy I bought this car from in '94 was a big-time soybean farmer in central Illinois, and had a 40x60 work building with all kids of projects-3GTO's, 2 Chevelles, a '55 Chevy, and restored old John Deere tractors, so no telling where his rig came from!
To the point, after it was all back together, with a Stanton 160 thermostat, it was not even getting 1/3 guage sweep warm, even on the highway on those hot days--I was very surprised and pleased too. I ended up putting in a 180 (even in the summer) as I kind of like to see the guage about half mast, and thats where it stays. My Judge is a factory air car too.
I'm running the factory 7 blade RA type fan, and 50/50 antifreeze.
Hope this helps with some ideas, I will post a coupla pics if I can get my wife to "learn" me how.
Good Luck;
Mootie