No leaks for 2 days, so here we go:
New plug, slightly different than original plug:
Original one was stuck, the 1/4 inch square bit just stripped it out. I used a slip pliers to remove it, but the metal pin that holds it in broke. It would go back in and seal ok, but work it's way back out.
(uh, the bottom is actually on the righthand side of the pic
)
Open the cap on the overflow reservoir tank to relieve pressure, then close it again to create a vacuum in the system. Then clamp the lower radiator hose (on the driver's side):
Remove original, install replacement. It was not necessary to drain the system in this situation, very little coolant came out as you can see: