I had the intake manifold gaskets fail on my engine, iron 289" with aluminum heads and aluminum intake at about 2500 miles.
While the engine was running I squirted
oil at the intake/head joint and could see where entire chunks of gasket, 1/4 to 3/8 had broken off and the vacuum had sucked it right in. Made the gap was obvious to see. Duh.
Upon removing the intake manifold, the Fel-Pro gaskets looked like they had been tortured...squirming around the intake/exhaust ports...have some pictures if I can find them.
I do not care what common wisdom dictates...they need to be retorqued...OFTEN...in my case often is every few thousand miles.
Anyway, I replace the gaskets...used the same Fel-Pro hiperformance whatever but have retorqued the bolts every 1000-2000 miles since...a few, usually bolts up front, squeak, creak and take another 1/4-1/2 turn to hit 22+/- ft/lbs...the others just sit there.
I am now at 13000 miles, about 6 weekend track events and no problems.
Next time the intake comes off will be to put in a hotter roller cam.
Oh, use more silicone than you may think to seal the front/rear of the manifold when you replace the manifold.
VERY HELPFUL...use four studs on the heads for alignment as you lower the manifold onto the heads...still be carefull the intake doesn't pull the gaskets "down" from alignment...but helps me immensely to align the intake and get a good seal.
Pete