Like Bill said. Check all the things. Make sure there is no air in the system (a problem in my car with the engine so far away from the grill). Make sure you have a good fan shroud, good thermostat, good water pump, fan belt, fan (turned the right way), all that good stuff.
If all that checks out, and she still overheats, then think about cutting the louvres.
The replicas had fake louvres and did not usually overheat; missing them is not your primary issue.