overheat only over 3000 rpm

diehard polylover said
therefore I did one hell of a cleaning, making it run with salty water for several days first it (it dissolves the limestone) , then with a mixture of water and baking soda after, (to take the acidity of the salt out of the block),

Salt water is basic, lime is basic.

You need to use a proper cooling system cleaner to dissolve any mineral buildup.

Timing will effect highspeed engine temp.

Water pump speed will effect high speed engine temp.

Automatic trans can effect engine temp

If you were in the driveway at 3000 rpm will the temp rise as described?