The fact of the matter is, that the 3.8L head/block design is hit and miss. You may be having luck with that set up, but the next guy can't keep gaskets in his engine with half the cylinder pressure.
The problem with these engines is minimal gasket surface area and uneven clamping on an already thin and prone to warpage cylinder head deck from the puny 11mm head bolts. Top that with the fact that Ford tends to load the accessories on the ends of these heads, making them even more unstable under load and it’s no wonder that over half the 3.8L's produced have had the head gaskets replaced at one time or another.
I'm not saying it can't be done, but it’s far from "bulletproof"