It sounds like you've got a rusted or broken-off stud. A little Kroil and some heat, plus a pair of vise grips will probably let you unscrew it, but don't break it off flush with the manifold or you'll have a devil of a time getting the rest of it out. Some of the older intake manifolds had a heat riser passage that exposed two carb mounting studs to exhaust gas, and they rotted off pretty regularly.
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!
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