When you replace the Head Gaskets.....you MUST replace the lower intake manifold gaskets......as they are NOT re-usable.
The lower intake manifold gaskets are much more likely to fail than the head gaskets on the 1996 3.8L windstar....they leak coolant into the intake....and it is drawn into the cylinders.
FORD has improved them.....so the replacment lower intake manifold gaskets should not fail again.
Also FORD has several TSBs that say NOT to use any sealers, etc on the head gaskets....they are to be installed dry.
They also say that you should NOT use any metal tools to scrape the mating surfaces clean......they MUST be cleaned but solvent and nylon/wood tooling is to be used.
You MUST be very careful to NOT make ANY scratches on the gasket seating surfaces....or leakage can result.