Your statement "Is there a thinner insulator possibly" suggests that your problem is that your studs are too short...NOT that the threads are mung'ed up. Is this correct?
Assuming your 216 is the same as the '48 216 on my AeroSedan, you can compare the protruding stud length as follows:
With my insulator mounted, it's just over 5/8" thick (0.640"), installed with NO gasket (it's original).
This leaves just over 3/4" of stud protruding (approximately 0.780").
This means that, if your manifold/carb are the same as mine, then you should have about 1-7/16" of stud sticking out from your manifold casting (almost an inch and a half).
Hope this helps.
