I have some advice after going through similar heartache this weekend with my 383.
First, I had a lot of trouble getting my headers to stay sealed, probably because they were poorly made, so I went to manifolds on my current build.
This weekend, I was filling the block with antifreeze in hopes of firing it up(no such luck) when it started leaking out of the end of the head. I thought it was a bad freeze plug, but it was a machine shop issue(who new?)
The most rearward valve cover bolt boss had cracked, and the shop installed what looked like a helicoil. Well, he may have drilled right into a water jacket.
My point is, and here is the fix: The machine shop recommended a permatex product called Hylomar medium blue: specifically for this kind of issue. It is formulated for high heat and chemical(read antifreeze) resistance.
We drained the head, and I glopped a bunch into the bolt hole and on the bolt, and waited a couple hours, even though it said it would take 10 minutes. I refilled with antifreeze, and no leaks, although It is not under pressure yet.
Having said all that, when my header bolts leaked, I did not use this stuff, but it is just a resource if you need it.