in theory 500 isn't too hard to achieve. think for the late 4.0 they quote 390 as standard. main figure i'm interested in though is the torque, although standard is quoted as 390 lb/ft, looking to get this up to around the 500 mark as well.
bigger bottom super charger pulley, smaller top supercharger pulley, decent exhaust, free flow cats, cooler stat, bit of work on the charger and sort the charge cooling out should see it to near as damn it 500.
am just waiting to get it looked at t see about getting some tubular exhaust manifolds made.
could always just bite the bullet and get a twin screw for it