Ya, I know the heat is in the top end and I'm guessing that water at the bottom of the block is pretty much not moving anyway from the amount of silty accumulation I've seen in some blocks.
You are pretty close in your thought process. The majority of the heat in ANY cylinder is the top 2 inches or so. If you have a 1/2 inch thick deck (or .750 with an X block or 1.0 with the R blocks) the very hottest part of the cylinder is solid. That's why they made open top blocks....upper cylinder cooling in long distance engines. There is very little heat in the bottom of the cylinders, so it's really wasted space. The factory COULD fill it with cast iron, but cost and weight make the bean counters lose their minds. But...you can fill up the space.
I use Hard Block about 90% of the time. I have used Moroso the rest. Never had issues with either. I am 100% sure of this because I have cut up several blocks (of the 3 major brands) that were filled and used up and never had any seperation or any other issues.
