I finally got the engine out of the van and removed the heads.  I must have done something wrong in installing the left head because the o-rings at the top of the cylinders were badly damaged.  I think it's safe to assume this was the cause of the problem.

The engine had good compression, and I can still see honing marks on the cylinder walls (at 120k miles), so I will just replace all the o-rings and gaskets and put the heads back on.

I plan on replacing the fuel lines, crankshaft oil seals and all the clutch components.  Anything else I should consider doing while the engine is out?  Heavy duty oil pump, perhaps?

Bill