When you're measuring do both sides . It's a known fact that the rearend could be offset by 1/4" or more. When I did my Monte, I used Billet Specialites Street Lites 15x10 5.5" and a Frame notch/mini tub because I had 5 1/4" on one side and 5.5" on the other from the face where the wheel bolts to the axle to the frame. Also from the inside of the outer wheel lip to the inside of the wheel house that actually stuck out farther then the frame, mine measured 11.5". Another thing to consider is sidewall width. I.m running a 298/55 that has a 12" measured sidewall. 12. - 10=2. / 2=1." sticking out past the wheel. So now I have 6.5" of back space, then add clearance say 1/2" now you need 7". Measure and then double measure. Tires and wheels are not cheap anymore. I know just bought 2 more rears for mine the priced doubled since I originally bought mine.
Here's a picture of mine