Of all the great suggestions you have received, one is missing. The water pump. Are you getting the proper amount of flow?

Also drilling a hole in the base of a 165 degree thermostat will not hurt, but will not cure an overheating problem.

When you add coolant it should be a 50/50 mix of water and antifreeze.

Good luck

Water pump is a possibility, but I don't think i've ever seen a car overheat because of that. Plugged up radiators seem to be the most common cause, rodding them out tends to only make them work well enough to make things harder to diagnose. I think after making sure there is enough coolant that a pluged up radiator is a likley cause. Sometimes a fan fails or an electric fan isn't coming on, but those problems don't show up on the highway. I had weird overheating problems because of a funky weber carb and a plugged radiator.