Even the 4-cyl guy, me, can direct you toward the investigation. The issue could simply be false torque on the headbolts. The 3.0L (6G72) has blind pockets for the headbolts and often bottom out. Especially if the heads have been milled. you need to measure the bolt length, measure the gasket thickness, etc. There should be room left over for bolt and often isn't.

1987 Raider - Roxy
1988 Mighty Max 2.6L Turbo - Pearl
1997 Mountaineer V8 - Freddy
2000 Excursion V10 - Freya