A partially or mostly plugged radiator (inside core) will overheat or run hot at hiway speed also and can be cool enough at idling speed.You burn less fuel at idle so less heat is spent ...hence if it is borderline it can still keep up.
BTW how HOT is HOT. I see a lot of guys who think 205- 210 is too hot. For a mostly stock later model this is normal. depending on your build this may or may not be too much.You need to verify temps with an infrared gun.
I drill any thermostat without a pellet if the cars hood profile is pretty low.I dont recall EVER drilling a chevy