My engine machinist warned me that he had this same problem in the past with Chevy L6's. He said it was from the assembly being so long, that the thermal expansion and then contraction upon cooling just literally "scrubs" the gasket apart. His advice was to use silicone lubricant (not sealer) on the gasket. Actually, I didn't use his advice: for 3500 miles before the rebuild, I had assembled the intake (aluminum) and exhaust (tubular) to the engine using NO sealer. And for 2000 miles since the rebuild, NO leaks yet.
