I have two tr3s that have two 5/16 holes drilled in the driver firewall for something. The holes could be for a windshield washer bottle, I am not sure.
My experience has been when the radiator is functioning correct with a 4lb cap, basically not plugged and clean, the radiator does not over flow.
The radiator would over flow if I filled it to the top, but I keep it about an inch or so below the top with no over flow.
Steve