overheat only over 3000 rpm

Hello Gang !
I'm back again.

The problem was finally solved !!!

I thank all of you for being so helpful with many hints and individual procedures for timing.
I agree with most of what you say,

Wonder what was the problem ?
I had modified the fixture that holds the radiator, putting 4 rubber antivibration supports, to avoid any future crack on the radiator body.
This simply pushed the radiator backwards 7/8".
Being the smart I am, I cut the top hose to macth the shorter distance but I forgot to do the same with the lower hose.
So the poor hose was pushed backwards and bent at a sharp angle..... and let it pass only a fraction of the coolant to the engine.
Obviously his quantity was enough to cool the engine at low rpm but insufficient at higher speeds.
I cut the excess of hose, restablishing the correct bend and everything now is running fine.
Now hitting the same stretch of highway where I had the problems the temperature stays at the right place all the time.
Problem solved ...

As for the timing, Of course I block the engine at 3000 rpm ( I have a tachometer) and there I turn the distributor to find the best point. Usually the rpm climbs to 3200 - 3300 when doing so.
Then as the member AJ FORMS also wrote in his post, I return to idle and I set the air screws at their best position. As you all know you have to play with idle screws and throttle screw up and down to find the correct mixture.
Then again, a little check at the distribtor position again to see if there is a change in rpm while turning it, and often I have to retard it a little little. THis often requires a little more adjusting at the three screws above.
Of course I have to be sure that the contact points are set correctly and everything in the distributor work as it should.
My final final check of the timing is is on the road. I have a vacuum meter on my cockpit and when you are on the highway at part trottle it shows immediately if the engine is running at its best or still need some adjusting of the timing.
Up to now I believe I have the best performance out my beloved 318 A Poly.

Cheers to everybody