Been reading and watching this thread...
I live in Fresno...we'll match or exceed anything Houston or Florida thinks is hot or humid. I can run the 482 high lift thumper in stop and go in mid-summer with nary a problem. More than a few folks watched at a SAAC in fontana a few years ago while I let the tin can idle for an hour to charge a dead battery...on hot cement after running a full session in July...and it never got past 85C. I run a Spal to pull, and two original style pushers in front from Shelby (which are more show as far as I'm concerned...replacing the air they block). I run 50W racing
oil with a cooler, and it barely gets above 120C. Yes, the aluminum block does help...but doesn't account for all that much given its state of tune. The headers were coated two years ago (for looks)...no difference from when they weren't before that. 160F thermostat, Edlebrock water pump, March pulleys.
If you haven't already...run straight water with a bottle of Water Wetter. Using more coolant is going in the wrong direction. Water is the best coolant, and the WW provides all the lubrication you need and makes the water a bit slimey to flow better.
On what basis do I suggest this? Autocrosser for over 30 years...where we haul ass for 1-2 minutes and then idle for 10-15 waiting for the next run, in the summer on hot cement. EVERYONE dumps the coolant and runs straight water, with a bit of WW in the last decade or so when it came out.
My other basis...as a farmer's kid. We always ran straight water in the trucks as we idled them along in the field to pick up grapes, etc.
{Edit...I went back and saw you did the full water replacement. Good, it's just Mustang's suggestion about running 100% coolant kept knawing at me.
}
Other typical precautions, of course...burp and reburp the system to insure there are no air pockets, check the air flow, dump the undersized pulleys and get the thermostat down to 160...get the water flowing sooner (biblically-speaking, heat begats heat), timing, etc.
I know you tried just about all of this. As you yourself have mentioned, it's difficult to only do one thing at a time to see what works. Lots of good stuff on this thread...all of it valid, but heat issues (like electrical) can be tough to resolve at times.
Given everything you've reported on...especially how the temp drops a bit when you give it some gas when it idles...I'd think about a bigger radiator, assuming no flow issues with the passages or pump.
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