Help with my first engine build (5.9 Magnum)

Have you considered grabbing a fresh set of hyperutectic pistons/rings to bump the compression ratio and improve the ring seal/longevity.

Don't hold you breath on grabbing some Eddie heads, as they are not flowing through the supply chain yet. Look on Hughes web site for new cast iron heads with better flow.

If you were going to EFI, there are some good tricks to improve the upper midrange power on that keg manifold. Those manifolds are unbeatable at low rpm power.