I'll agree with the water as being the best chance of being the culprit. Since the 428 is a 332/352/390/406/427 in disguise, there is a distinct possibility that the nasty intake/head gasket interface may have a small leak - which would not show up in a compression test. If you have an aluminum manifold, and I assume you do with dual quads, there may be some erosion or corrosion and allowing some water to leak by. If you are using plain water without antifreeze, the pH can go or be "off". If you have anti freeze, and if it's old - same thing. Acidic by products of combustion will eventually seep a bit into the cooling system, high pressure to low, regardless of how good a set of gaskets you have.

Think positive - if it's not a cast iron manifold, at least it wont weigh 87 pounds and require 2 large men and a small boy to lift off - been there.........