Another "Hot" problem

the clutch fan seemed to working

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is not good enough.
Shut the engine off at 200 or so. grab the fan and try to turn it. It should be quite difficult and the belt must not slip. A thermostatic clutch is the best,IMO.
I run an anti-cavitation plate on the back of my 8 vane waterpump. My rad is a stock A/C teener from 1973, and ASAIK,has never been touched. I have owned that old-timer since the mid to late 70s.I did cut out the core support to take advantage of the 26 inches.
If you are overheating at over 50mph and over 2500rpm, then there is a real good chance your rad done, or your teener has problems and is creating more heat than it outta be.
If you are also overheating at idle, then the entire cooling system is suspect, and again, the teener may be creating excessive heat.
Since the engine and the rad are both suspect at this point, I would be verifying that the timing is close,doing a blow-by test
and finally,a compression test,in that order.
Oh yeah, and blowing the mud out of the rad fins. Stuck behind the A/C condenser, they both have a tendency to accumulate a lot of dirt.I'd do this first, for obvious reasons. I usually take the rad out, so I can clean the condenser out first.