mrapii said:...G-Bodys need a 4.5" backspacing to not run on the frame.
If you are talking in the rear, 4.5" is too much from my experiences (and even moreso up front), depending what size tire you want to run. With 4.25" BS 80/81 Z28 wheels I couldn't even mount the wheels with 245/60/15 rear tires. A 5/16" spacer allowed me to fit them with minimal clearance. Up front I still had just a touch of rubbing that the wheel weights would get slightly shaved - 215/65's were tight around the wheel well. I now have 245/60s all around on stock 3.75" BS Monte SS wheels.
Most G-body stock backspacing ranged from 3.75" to 4.25". Front BS clearances change quite a bit once you go to larger wheels.