Brian, Coach Rocket here:
I reckon you'll need about 10 reps at 315 to equate to about a 405 single. But since we're going for 500, I'm going to need you to pull at least 20 (my one rep max calc says 23 reps).
So if you don't mind, Brian, we're going to need you to not stop at 10 on the 315. And if you think high rep doesn't work on deadlift, you'd be wrong. It's actually extremely effective at pushing strength up.