The 255/45's should fit on an 8" rim, but 9" would be better.
As far as fitting, as others have already mentioned I run 17x9's with 275/40/17's on my car with 5" backspace. Initially they rubbed slightly on the passenger fender lip, only when turned and going through a dip (like pulling into a parking lot). After a little fender lip trimming and increasing my camber to -.9 degrees I don't have any rubbing on the fender, and I've never had any issues with the frame or even the fender on the driver's side. I'd love to have a little more backspace, but there isn't much room with the inner tie rod end. On my car, I think about 5.125" of backspace would be absolutely perfect, but I was fortunate enough to be able to correct with some additional negative camber, which doesn't bother me as I run the same size tire all the way around, so I can rotate tires.
Some of it will depend on your car, they're not all the same, and even an 1/8" can make a difference. A 255/45/17 will also be slightly taller than a 275/40/17, a little less than a 1/2" depending on the tire manufacturer. Depending on how low your car is, the height may be more important than the width. I know on my car the 1/2" taller tire, even though its narrower, might give me some rubbing issues.