Quarter Mile Gear Selection

I dont think having a pro gear would be required. Standard gear in that S60 can take a lot of abuse. Had that same setup years ago making right around 700 horse and using a brake quite often, never failed me, used 4.30 gears.
Hard to know what rpm the car will trap at without more info than what is thus far known. Slip, car weight, etc, etc.
But I would agree with others, 4.10 would definately be a very good place to start. Going to a 4.30 should raise your trap rpm roughly 250 or so more rpm over the 4.10’s.