It looks to me like the hard work is on the manifold..I think you will find what Don suggests above...gunky leftovers of bolts in the holes in the block. I'd poke around in those holes with a pick and then get the 3/8" tap on that bad boy (or "thread chaser" if you have a set..I think they do less damage to the block than a tap when you trying to simply clean up an existing threaded hole)
I bet if you can get the gasket off the manifold, the nasty old bolts will come out. I am trying to figure out what was holding the manifold to the block..probably the rust!
Both halves of those mating surfaces do need some cleaning up though.