The spring goes in first, then the valve, then the allen plug last. I don't think your block has the "mystery hole" either. But if you did it would be affected by the adapter plate or later adapter
oil passages bleeding into it. That dang hole was on a old 406 block I had also. I like Chanmadd's idea of trying just the stock filter and adapter...and also going to a little heavier
oil. I'd document each step one at a time instead of doing a combination of things at once. I would also try the simple things before digging into it.
G.