[Tommy]

I'll add my personal anecdote just in case someone finds it useful. After rebilding my 351W with aluminum heads, the stainless steel headers on one side leaked after a few hundred miles of use. When I replaced the gasket, I added a generous bead of silcone sealant. I now know that the relatively low torque on the header bolts combined with the thick sealant to allow the headers to stand out from the face the heads. When I started running the engine, the headers rocked back and forth on the bead of sealant and quickly began to leak.

Faced with headers that leaked with two different gasket/sealant combinations, I investigated further. I took the headers to my local machine shop and asked them to check the mounting surface for flatness. Sure enough, the mounting surface was warped. I had them mill the surface flat and remounted the header with a normal gasket, normal torque and safety wired bolts. It hasn't leaked since.