If they are the rings shown by 8.5010 in the page from my 1955.2=60 GMC Master Parts Book, making your own is easy. Like Otto says, they are designed to align the manifold to the head. I had 2 and needed 1. My FLAPS has a selection of short exhaust pipes, some straight, some adapting some diameter pipe to another. I found one that was just a little larger diameter than what I needed, cut a short piece (make sure it is short enough that the pipe won't keep the manifold from tightening up to the head), then cut a slot across the band so you can fit it tightly into the head.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
