It turns out that I apparently installed the two intake manifold gaskets in the wrong order. I guess I wasn't paying enough attention when I took the thing apart a couple of months ago. I then removed the intake manifold (which required a lot more work than the Ford workshop manual indicated), installed the new gaskets in the right order, put it all back together, added water and then pressure tested the cooling system. No leaks.
So far so good.
Paul B-West LA