Thanks for the tip, Gary, I'll look for the kit.
What still bothers me, though, is being certain I have completely DRAINED the system before adding pre-mixed coolant. I don't want to dilute with several quarts of residual water, and my normal method of adding concentrated antifreeze to the 1/2 capacity point - then finishing with the pre-mix - didn't work on this car. Apparently quite a bit of water remains in the block and heater system. I did not remove the block drain plug - maybe that would help - and I made no attempt to get the climate control to open the heater valve.