Oh, I thought you bought it from Tom directly. That would have helped in this case, but you could call Tom and ask him, I suppose. I think it should had a flat round gasket, too. But that gasket is only about 1/16" thick or less. Here is a link to Tom's website:
http://www.langdonsstovebolt.com/store/online-catalog/#!/Stovebolt-Mini-HEI-for-GM-216-235-261-&-GMC/p/1222040/category=18665949
