another 360/408 over heating

Here's your test; (short version at the bottom)
I would
Make sure the Distributor has a working Vcan on it, and that the trans/TC are working properly.
Then I would
remove the fans, remove the shroud,remove the stat and install a gutted one,dump the coolant and install straight water, swap on a 7psi cap. Then I would start with a cold engine, point the car down a deserted road and drive it at 30/35mph, and see what happens.
I would be prepared to camp for an hour if it gets hot. Then I would turn it around and go home. If it overheats at speed now,and the cooling system is not taking in combustion gasses,then; first I would make sure the lower hose is free of obstructions and the anti-collapse spring is in there. Secondly, I would remove the water pump and make sure the impeller is not slipping and that it is rotating in the correct direction. If that is good, then, thirdly; I would remove the rad and drive over it a couple of times.Ok no, first I would prove that water is not getting thru the rad fast enough and that the hoses don't still have shoprags stuck in them, as some guys do;then I would drive over that rad.

Here is your rad test; with the new rad vertical as installed; the water should run out the bottom at least as fast as you can fill it from the top by pouring from a jug. In other words you can never fill it up with no hose on the bottom, no matter how hard you try.
You can time it but do it right. Time how long it takes to pour a jug in the top, and time how long it takes for the water to come out the bottom.... from the instant it first starts running out; not from when you begin to pour. Use discretion when to stop the timer; I mean don't stand there 10 minutes waiting for the last drop to fall.
I guess you could just do the water test first and save some diagnostic time... lol.
When you get this sorted out, now you know your cooling system is up to the task.
But you still have to solve your low-speed airflow problems.

I don't know much about pump speeds or too-fast water circulation or how hot is too hot, but I know of a guy who does and he has a lot to say about it. His name is Smokey Yunich. His book has a whole chapter, that helped me build my powerhouse 367.