The rings don't do anything to seal vacuum- - - - -they're only there to keep the intake ports in the head and the manifold runners aligned correctly. I have made rings from slices of exhaust tubing with a diagonal cut to adjust the circumference of the ring to the correct size to be a snug fit in the counterbore. Some online vendors sell stepped rings to adapt a 216 manifold to a 235 head which has larger ports. You can make longer pins by cutting a 3/8" bolt off to fit the threads in the head, leaving the unthreaded shank a little longer than the original part.
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!