In simple terms this is my understanding. The threads into the alloy block are all UNC(fine threads are no good in an alloy block). Threads into steel are all UNF. Then circa 1978 metrification (those who had a 109 v8 might relate to this) started so any new parts were often metric I think it might even be explained in the workshop manual for the 109v8. Obviously the blocks could have been metricated (with course metric threads) but that would be confusing right and really mess up some blocks? So this is why you find that parts on the later bodies are often metric and parts on the engine are imperial. It's useful to know this.

 

I can’t find this anywhere, what LENGTH? Someone previously said 3/8-16 but left out the length spec.

Posting here as it’s the best thread I’ve found online about these bolts

 

Not sure. Not to be rude, but rather than asking on the internet, I would pull an adjacent one and measure it. Or if you are concerned about un-threading another one, stick a skewer down the hole and measure the depth, and go shorter than that by about 1/8". Or if the whole manifold is off and all of them are broken/gone, check the hole depth, flange thickness and spacer thickness to find the length.