When you say, “…the middle main bolt seems to not want tighten up like it should on one side…” do you mean is keeps turning when torqued or the bolt doesn’t come in contact with the main cap saddle? Is it possible that there is something in the bolt hole? Can you take the cap off and look down inside the bolt hole? I’d try chasing it with a thread cleaning tap and make sure you’ve got the correct bolt and give it another go.

It takes a bit to bugger these holes but it does happen and then it is best to have the machine shop examine and rework it. If you’re going to build a 383 the block will need to go to the shop anyway to grind the proper clearances for the longer stroke.