It must be different on the AP5 as it is on the drivers side just behind the distributor. I couldn’t budge it, and I was worried about snapping it off.
@dodgedart1968 thank you for asking this question, I spent quite a bit of time trying to find the plug yesterday. I have an overheating issue on a 1963 AP5 and I really want to ensure I get the cooling system as clean as possible.