The rings are for alignment of the manifold to the head, and they're essential for a good gasket seal. The rings don't seal vacuum but they do allow the manifold and the head to mate at the proper position. Just measure the depth of the counterbores in both items, and adjust the thickness of the rings accordingly. "Measure twice- - - -cut once!"
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!