Just got a SN89 head unit w/6 rib pulley and I just want to put out low boost. The pulley on the head unit measures 4" o.d., is this a stock head unit pulley? Would I also combine that with a 6" crank pulley to keep boost somwhere between 5-7lbs. ? Would these 2 pullies keep me at safe blower rpm levels? This is going on my 93GT mustang w/ported stock heads,Typhoon upper/lower intake 65mm tb, 73mm maf, 30lb injectors, Steeda 18 cam.