I'm working off memory here so it might not be Gospel, but I think the 3.5" backspacing allows a 10.5" wide tire to "Just Fit" inside the wheel well. I dunno how tolerant of body sway they will be but I'm pretty sure you've got clearance.
I'm also pretty sure that the 3.5" backspacing is a stock wheel dimension. I have 15 X 8 X 3.5 wheels on my '34 Ford and the tires fit perfectly under the stock rear fender. On a Chevelle, you should have another 3" outer clearance if you use the same tire/wheel I'm using, so logic dictates that your 10" tire and wheel should fit.
Hot tip is to find a buddy who has a set of any 10" wide X 3.5" and do a trial fit. It might save you some shipping fees . .