After posting I went back out to the shop and decided to just pull the manifold and run it through the sandblaster - so I got a good look at what's underneath. Also, I got the nuts off with a little more coaxing so all is well with that. Turns out only two of the bolts in the picture secure the plate - not four. The other two are manifold bolts - which I found out the hard way when the manifold wouldn't budge after I removed all the other bolts. There are two just like it on the other side of the manifold. But they are really deceptive. They have a smaller head and are at an odd angle compared with all the other manifold bolts. Anyhow, looks like I''m out of the woods on this one.