the "several carbs" he has had on there would rule out that, I'd think.
No 2 carbs I would think were jetted identical. especially that many carbs taken over that many model years. IDK if he tweaked them out and made all the adjustments possible on each carb while it was on this engine, but unless they were all Holley 2 barrels that came off of '85 Dodge trucks, I would have thought that he would have seen something different in how hot it ran between the various carbs if this was the issue. I have installed headers on more than 1 vehicle myself over the years and have never had it cause a lean condition or an overheat; headers release alot more heat than manifolds, if anything I would think things would run cooler with them especially at highway speeds, that was why so many motorhome guys would go to headers to dissipate the heat better;

Having said that, I used to maintain a '73 Dodge Van for an older couple in town, they'd bought it used and had sat more than not, before they got it; back in like '95 the thing had like 32K miles (over 20 years old "then") One time I got a call for a water leak, the back freeze plug on the pass side was trickling water; I went to replace it and that remains the most gunked up motor I have seen to date; I could not believe all teh rust and scale in the water jackets; it was so bad as to actually help slow down the leak at the freeze plug. They always only drove it in town and it wouldn't overheat but did seem to run a little warmer than it should; good thing they had not taken it out on the road.
Having discovered that, I popped every freeze plug I could and flushed the $#!t out of that block. It made a huge difference. Trucks and vans have plenty of room to get at most of the freeze plugs.
You don't need to pull them all but at least the easiest ones on each side and run a garden hose in one so the water can run out the next freeze plug hole.

Also; something I have seen more than once is casting sand remaining in the jackets after many years even with multiple "typical" coolant flush jobs over the years, sand collects in the very bottoms of the jackets and coolant just flows over. I just pulled a '76 360 apart 2 weeks ago, and sent to the machine shop for hot tank and bore/hone. I popped the freeze plugs from that block on the stand, and found it to have ALOT of sand in the jackets; this engine came out of a wreck 20-some years ago and put in a barn with a tarp over it and basically forgotten about.
I found the 4.0 block on my Wrangler to have had the same issue when I pulled it out to replace the frame 5 years ago.