the octane diffeence is the way it's being measured, many different methods
Octane rating - Wikipedia, the free encyclopedia
i'm a little confused when you state Open chambers @ 67cc's, are they milled quite a lot?
Open chambers start life in the 76cc range and have a full round shape to the chamber
long rods in a 351C isn't gonna make a hill of beans worth of difference in the real world, and we're only talking about a 1/4" or 6mm longer at that. unless you can find a great deal on the right pistons and those are the only rods you have... just build with stock dimensions.
why Hypereutectic pistons and not Forged? spend what you were gonna on the lower compression height hypers on better metal!
i don't see a 1.5mm dome affecting flame travel much at all... ?