It's a pretty simple task to narrow your home-brewed rings a little, but yes, replacements are available. There's even a set of stepped rings for fitting a 216 manifold to a 235 head, which has different manifold and head hole diameters. I'd use my lathe for fitting purposes, but a vise and a file will also work if you support the rings with a piece of dowel rod or something similar to keep from collapsing it while you file or grind the thickness down a little. Try some of the antique parts vendors who advertise here.
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!