Wheels are pretty big. Fronts are 18 X 9 w 5 in bs with 235 40 tires. Guy at shop telling me to fit I need to go 17 X 7 w a 4 in bs. I really don't want to go to a 7 width.
Not specifically speaking camaros here, but a 5 inch backspace on a 9 inch wide wheel I believe /should/ be a 0 offset. 7 inch wide with a 4 backspace should also be a 0 offset. So your offset isn't changing, just the width. So technically, if you're moving from an evenly spaced wheel (offset hasn't changed) each inch you go is about 25mm width in difference (rim, not tire).
Where are the tires rubbing? I don't know enough about the fenders on the camaro to know "what's normal" to rub, but if it's just the lip rubbing on the outside of a tire, or the tip of it you just need to think of the backspacing as even.
Basically for every inch wider you go on a wheel on a perfectly 0mm spaced wheel, you're increasing the width each side from the center half an inch. So if you go from 7 inches wide to 8 inches wide, you're actually going .5" on the inside and .5" on the outside incrementally. So if you're rubbing by "just a little" or "just barely" you may not have to drop down a ton unless you're rubbing a ton.
Also tire size and profile will have a lot to do with it too. My daily was rubbing the oil return lines because someone put 245/55 on the rear, I moved down to a 225/50 and it made a HUGE difference in clearance, same rims. All the clearance in the world now.
Here's an awesome wheel calculator to play with, it'll give you an AWESOME visual of the changes you're wanting to make.
Just change the size you have on the left or want to, and then put what you had before on the right and you'll get a visual idea of what is happening. Really cool program.