/6 overheating need help.

Well, it sounds like it never got heat loaded like it is now with the AC and no shroud and summer temps. So it is back to the basic questions of:
1) Is the radiator sealed against the core support so all the high speed air is force through the rad core?
2) Is the spring inside the lower hose o prevent collapse/partial collapse with higher rates of pump suction when running/cruising?
3) Is there any clogging in the new core due to loosened crud blocking it up?
4) Is the thermostat in the right way, spring towards block?
5) Thermostat tested? They can be bad out of the box...

BTW, the fan shroud is out of the picture at high speeds so I would not focus on that.

If all above is good, then before changing the rad core, I would check the cooling system for the presence of combustion gas with a pressure checker or CO2 test kit. A small head gasket leak or crack somewhere will cause these symptoms.