This will be a major project. Before I spent any time on it, I would pull the pan and see what the pan rails look like and the bottom side in general. Then I'd pull the heads to see how bad things look. Decide if it is even worth screwing with.
If so, then I'd take a drill with a wire brush and clean all the block areas that you can between the valves and the bores. If you find obvious cracks, then you can decide if you want to go further. My guess is that at the very least you'll be doing some "cutting, drilling and smacking" to get it apart . . . be prepared for a LOT of work.
