If you look at Sewell's website ( http://is.sewellparts.com/accessorie...2011/1800.html
), the f-sport springs are for both RWD and AWD, so my guess is they would fit.
Are you sure your 350 isn't already on optional 18s or are they on the stock 17s. It seems like there are more ISX50s with optional 18s than stock 17s.
For future reference, if you put on staggered wheels on your AWD 350, make sure the spinning diameter is less than 1% of a difference between the fronts and the rears to prevent future damages to your drivetrain.