because they arent GNx rims , they are GTAs
and how much of a spacer?
GTA rims were designed for an F body firebird and are backspaced and stamped (on back) accordingly ' front only' or 'rear only'
front gtas have a different backspace than rear gta rims
with 255/50 mt radials and GTA fronts mounted at rear on a gbody they may need a thin 1/8" spacer , really depends on if axle is centered and alum or steel drums
with rear gtas mounted on the rear of a gbody the spacer will need to be about 1" thick or even larger
also rear gtas can be mounted on the front of a gbody but will require a spacer of approx 5/16" and longer lug studs or use of et cone lugnuts