Those rings are there to make sure the intake manifold and cylinder head ports align properly with the gasket. They DO NOT seal vacuum- - - -that's the gasket's job. Be sure the rings are short enough to allow the gasket to be compressed properly- - - -if the manifold has been machined to correct warp, it's possible for the rings to bottom out in the counterbores and cause a vacuum leak. If they're missing, you can make new ones from a thin slice of exhaust pipe tubing- - - -just cut a diagonal slot in the ring and trim the ends until the diameter is right for a snug fit in the counterbore. If you happen to be running a 216 intake on a 235, or vice versa, there are stepped-down rings available from some of the resto-ripoff vendors to align the mismatched port sizes.
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!