While I was taking the radiator off, I (obviously) needed to drain the cooling system.  Unfortunately, the petcock on the engine block didn't seem to be there.  The manuals all just say it's on the lower right side of the block--that's still a large area.  Someone in the past has removed it and (presumably) plugged the hole with something else.  The actual location depends a bit on the actual casting number of the block--there were a few variations in the L-134 over the years.  I'll have to scrape grime to find the spot, and probably remove the alternator, too.