It seems to me they are 3/8"-24 thread inverted flare plugs. My 81 Delta 88 and my 82 Delta 88 parts cars came with factory plugs with sort of a nipple on the end. In 83, even without the CCC we got air injection on 307's. I actually ground the nipple off, not realizing I needed to pull tubes in the head on my 83 307 that was transplanted in my 88 Cutlass to fit them.Don't know the size exactly, but they aren't metric, and they are NOT NPT threads. Think how the brake lines are. Inverted flare fittings. Straight threads, but not sure of the size. Why would you need to re-thread them anyway? Just get the right size inverted flare plugs and you'll be fine if that's what you want to do is plug them off.
TBH, I never measured them. Someone else may be able to shed light on the size. MAYBE 3/8"-24 thread or 7/16"-24 thread? I know it's SAE.
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