408 Magnum stroker running hot.

I think you have done your due-dilligence, and the rest of the guys have all made good points; so I'm gonna take a different direction.
First a couple of questions;
>Are you running "vacuum advance" or it's equivalent?
>What rear gears, or what rpm at 50mph?
> where did you degree the cam to?
At 50 mph/2000rpm, your engine will want in the neighborhood of 50* ignition timing. At 60/2400rpm, she might be wanting 56*.

If you don't supply the engine what it needs;
the A/F charge will not finish burning in the chamber. And the energy of the burning expanding gas will not ALL be transferred to the crank; a bunch of it will go straight into the headers. More on this later.
But with this retarded timing, the engine will be down on Part-Throttle power, so you will have to drive deeper into the throttle to maintain your chosen speed...... which exacerbates the first problem.
Furthermore, the still burning gasses leaving the chambers, heat up the exhaust ports. Guess what is right above the ports; hyup the water jackets. So you are unknowingly lighting a blow-torch right there in the ports. The energy in the fire needs to stay in the chambers for as long as possible, to push on the crank for as long as is possible, and to cool off in there. Cuz from physics class when you compress a gas it heats up, and when you decompress it, it cools off.
To continue; that 3200TC is not locked up at 2550rpm, and will be slipping maybe 10%, could be more. That slipping generates heat; send it into a stand-alone temperature-controlled aux cooler.
To continue; You better have fresh air to the carb/throttle-body. Feeding your engine 300plus degrees air is like feeding a runaway nuclear reactor.
To continue; Where does the exhaust dump?
At the back bumper,at speed, the car usually generates a low-pressure area. You want the exhaust to dump into it, so it will help pull the hot exhaust thru the mufflers.If your pistons have to pump the gasses out, that is gonna take additional energy away from the crank..... causing you to add throttle.
But worse is if the pressure in the headers is higher than in the intake. This "almost never" happens; but if it did, the headers would send hot exhaust gasses up into the intake thru the overlap period........ and those hot gasses are gonna go where? Hyup straight into the chambers when the intake opens.
Finally, with retarded cruise-timing,(sub 50*), The exhaust gasses will finish burning in the headers, causing them to run very hot. But worse is, that these gasses are now occupying space, and time, and hanging around, instead of zooming on down to the merge, the way they are supposed to, and so the headers cannot perform their first function, which is to produce a low pressure area behind the closed exhaust valve, which is supposed to "jerk" the exhaust into motion when the valve opens. So now, instead of the exhaust hurrying on down the pipe, it hesitates there, cooking the water jackets.
I got more but that'll keep you busy for awhile.
The point is to put some stinking cruise-timing into her.

For reference, my Eddie-headed 367also at 11/1,
likes 60 degrees@2240rpm/65mph in overdrive (manual trans). But I don't always give her what she wants,lol.

If all else fails, I had to take my engine apart and loosen up the KB107s and increase the top ring gap............ which finally cured it.

I took this direction because you said it cools off at idle with the vehicle stopped. This is the hardest test for the cooling system, and yours passed.
The faster you drive, the easier it is supposed to get as ram-air cooling takes over, and the water-pump speed is rising, and the Ignition timing is coming on line. This is where yours is failing.
The other guys are steering you right as to the cooling system, so I thought I'd go this way for a change,lol.
I'm not saying I'm right, just offering an alternative.