If I had water in all 8 cylinders, I think I would be looking at something that could get water to both heads - like the intake manifold. I would be surprised if both of your head gaskets went at the same time. You might have exhaust manifolds that had frozen and cracked internally, but I think it would have shown up before 10 hours.
I don't think enough water could get past the raw water cooler to blow gaskets in an engine when there is a hose to each exhaust manifold that can handle that much water.
'96 ProStar 205 SD LT-1