i think you're picking fly **** out of pepper here.
you're not building a street/strip motor, you're not doing burnouts, you're not hunting 10th's or trying to eek out MPG.
the
stock magnum bottom end has proven itself both capable and durable enough for your application, why not go with that? one of the benefits of the stock mag pistons is the design: the top ring land is nowhere near the crown and the bottom is fully clear of the pin boss. on those 405's the top is clipped by the valve relief and the bottom shares with the pin boss. not for me, so siree, not on a build like this.
you're worried to death about the CR, yet you only want to run 87. i'd stop fretting about it. also, the newer motors were built a bit better than the older stuff with better materials, better machining, tighter tolernces and more attention to detail.
i think
@TT5.9mag gave some solid advice. screw it together stock, bump the cam up one size with a
custom grind (don't buy no junk off the shelf) get a good dual plane manifold and roll.