This is a Mega Block, the area in question is between #2 and #3, that area should be solid!
I'm thinking you must really mean 2 and 4. Or maybe 3 and 5.
Given the odd pattern of corrosion I wonder if the bores were straight from the get go. Although we will never know the answer, It's safe to say they're not going to be straight now. If the corrosion got to the rings, looks like there is a good chance of that, then safe to assume they're gone.
I suggest check ring side clearance and overall condition of the ring grooves before putting pistons up for sale or deciding selling price, (to avoid angry future customers). Good chance they are still OK but never hurts to make sure.
7.10 rod and 4.50 stroke is probably the path of least resistance in terms of not only known decent rod ratio (Same as a Pontiac 455, can't get much more conservative than that) and relatively quick/easy parts availability.
