Since I last posted I have soaked the rusty bolts 3-5 times a day in PB-Blaster. It was really really bad and the bolt heads were really gone. I could go into detail but what I ultimately ended up doing was working very slowly focusing on one bolt at a time. 6 of the intake bolts had the heads literally twist off with hardly any effort. I used a sawzall and 4 blades to cut them from the heads.
Below are two photos of what the 400 small block engine looks like with the intake manifold and heads removed. As you can see mice decided to build a home inside. I checked the cylinder bore with an inside micrometer and it has never been bored out.