As far as I know you cannot have boost lower than what your wastegate spring is set for. Unless you can figure out a way to make the wastegate open sooner.
I would probably just purchase or borrow the proper spring for 6 psi to be sure you'll have the proper boost level.
2004 SVT: Boost soon!
2010 Ford Escape 3.0 V6
2004 F350 6.8L V10
1986 XR4Ti 2.3L Turbo, 15 psi
2007 ST ; formally bewst'd - R.I.P