Your radiator can either provide sufficient cooling or it can't. Drilling holes in the thermostat will accomplish nothing. If your thermostat is opening at 80c but your water temperature is over 100c you either have a coolant circulation problem in general or a blockage in the radiator. Drilling holes in the thermostat will allow "some" circulation when the coolant is below 80c. Inasmuch as your coolant is apparently never below 80c (after warmup) then drilling holes is a waste of time.Everything is new. Gaskets are not leaking. I have an 80 degree thermostat right now, can't seem to find a 70 degree anywhere. Drilling holes along the perimeter of the thermostat will allow more coolant to circulate , at least I think it will. At this point, if flushing it doesn't work, I'd rather drill holes in the thermostat than run without one. The only other possibility that I can think of is that it's running too lean. I haven't checked that yet but it was super clean when I just had it smoged.
You can try an infrared thermometer to see if you have any hot or cold spots in the radiator, but being that you just had the radiator re-cored I'd guess your problem is circulation and not the radiator.
P.S. Retarded timing can cause overheating. Did you check the timing? Also a collapsed lower radiator hose can cause overheating.