Well I feel sheepish, or perhaps blind.  The petcock was indeed there, behind the alternator.  It was covered in grime, but I used a better light looking for it last night.  (The garage isn't exactly well lit...)

Here's a picture (of someone else's block, found online)showing where it is on this block--the casting number on this one is the same as on mine.  Look to the right of the bottom of the oil filler tube.

I had to pull the alternator to get at it.