I'd start with 15 and see how that works. With your kinda power, you'll need to slip the clutch to keep from roasting the radials unless they're prepping the track very well.
I'd try launching at 4000 to start and if you bog, raise the pressure a pound, spinning, drop it a pound or 2. You really don't want to run radials with too low pressure, you'd be better served with a slick in that case.