Professor,
I'd say you're gonna have to be really quick if planning on doing this without losing a bunch of coolant. Even before you get the plug all the way out the coolant is running down your hand and tools. Make sure you do it with a cool engine because I'm thinking while you get the plug out for those seconds before you can get the heater in you may want to hold your hand or something over the hole (it comes out quick). Plus the time it'll take to get the heater seated properly and threaded to the point of non-leakage, ya better have some extra coolant on hand. My plug is a 14mm hex if that helps any. If the heater is like mine you'll also need a large deep socket, mine is a 27mm. And also I went without silicone or tape on the threads. I was skeptical but I had some silicone on hand just in case and ended up not using it (no signs of leaking). Although with the silicone, removal will probably be easier in the future. Too bad you're not "around the corner", I'd trade ya my heater for your plug if all was the same size. Glad to be of assistance. Billy