Originally Posted by Uncle Ben
How clean was the old coolant? I ask as the older SB's had a tendency to accumulate all the rust and corrosion from poorly maintained cooling systems around the rear cylinders. Once it builds up it restricts or even closes off coolant flow to the heads at the back port. Not saying this is what's going on but if it is it's really hard to get that crap out. I have seen and dealt with this a few times over my past life as a shop slave. It was really bad with the '60's SBC. The 400 ci has Siamese cylinders (they touch each other so no flow between cylinders) and the heads have steam holes to allow flow which can have many restriction issues.
And that's ultimately what I wonder is going on. When I pulled the coolant with the first water pump change when the heating issues started it was pretty nasty ... dark brown/blackish.