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
