Hi Bob,
You are correct about it should be 5/16" not 1/4". The proper bolt is 5/16-18 x 3 5/8 long.
If your intake has 1/4-20 thread it is one of the two things you mentioned.
02/22/71 was a Monday back in 1971.... Machinist may still have been suffering from the brown bottle flu Monday morning from the weekend. Hard to think it though. You sure it's not just a banged up starting thread on your intake? Or does a 1/4-20 work just fine. If so, I would get me a 6" long 5/16 bolt, cut it to 3 5/8 and have a machinist cut it down and put a 1/4-20 thread on the end of it. No body will ever detect it. Or install 5/16-18 helicoil in the intake. Guess which ones easier!
Good Luck
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