At work we sell street rod and drag race hubs that use Holo-Krome SHCS 1/2"-20's for wheel studs. I would suppose that you can use 5/8"-18 SHCS' for your application.
Have you called ARP and asked them ? They have a lot of stuff they do not put in their catalog. They will also make custom fasteners. I was surprised at how reasonable they were in making wheel studs for the Elva Mk VIIs that I worked on. That is not to say that they were cheap though, just reasonable.
"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages