The higher the offset, the more the wheel sits inside the wheel well. Stock on our cars is +55. My 18x7.5's are +45, I have no rubbing. When I jack up the front of my car, the rear tires tuck. But I have a little uncorrected negative camber that helps tuck a little bit. I'd go higher offset if you can, I know my buddy rubs a little with 42.

But I can't say for sure with an 8" wheel and 40 offset...my guess is you'd rub. Anyway you can have a similiar wheel bolted up to your car to check?