The little holes might help if you've got air in the system that won't bleed off. That's the old racer's trick to bleed a cooling system of air quicker.

I've got a healthy 355 with an Alpha gen 2 drive and it runs 140-150 all the time.

Bruce Wolff (marine engine builder) just built a pair of small block 383's for a guy in Chicago for a 27' Magnum. I think it does something like 78-80mph. He has done several 383's. I'm sure he could answer some of your questions. His shop is in S.W. Michigan 616-422-1090