Hard to say? The rotors your using are almost an 1" bigger than mine? It should fit but that's only a guess? I doubt a 9" rim will fit even with custom backspacing if everything is stock ( fenders etc...) but I don't know for sure.
A 9" rim will fit just fine. I have 17x9's all the way around on my Challenger, with 275/40/17 tires. Backspace on my rims is 5", same as the backspace on the 17" year one rallye's. I had to do some fender lip trimming on the passenger side to clear the 275/40/17's, but with a 255/45/17 you'd have plenty of clearance even on an unmodified car.
Yea... That's the problem...currently the 17 X 8's are not available in the Magum style, only 9's.
Also just noticed that the Magnums have more backspacing then the Ralley rims 
on the same width rim ? Is that a good thing or bad ?
It says that the 17x8" Magnums are being retooled, so they will be available again in the future. No date given, so I suppose it depends on where you're at with your project.
More backspace is a GOOD thing. 5" of backspace will work in the front, but I had to do a little fender lip trimming on the passenger side fender to get my 275/40/17's to work. 5.125" would be perfect. Any more than that and you'd run into tie rod interference problems. With my 17x9's and 5" of backspace I have about 3/16" to 1/4" to the tie rod end, but that may vary a little with the style of the rim lip.
BUT, most aftermarket disk brake kits increase the track width of the car. Dr. Diff's kits move the wheels out 3/16", most of the Willwood kits are about the same (not all, some are more, some are less). So having more backspace is usually a good thing. You can also add a small spacer if you have too much backspace, anything less than a 1/4" is no big deal. But if you don't have enough backspace you're just out of luck, so having a bit more is good.
As far as the rotor clearance goes, that's always something to look at when getting aftermarket rims. Part of it is the size of those brakes, but the calipers matter too. Wilwood calipers might clear where the Viper calipers might not, or vice versa. Could even depend on which spindle was used. As far as the Viper calipers hitting, it was on the inside of the rim lip, not on the spoke part of the rim. So, something smaller than that 13" kit might be ok. But since the wilwood kit uses different calipers, all you can do is go to wilwoods site and look at the diagrams for wheel clearance. Or try to find someone else running big rotors with those rims.
Here's a look at the passenger front fender on my car with the 17x9's with 5" backspace and 275/40/17's. My car also sits pretty low, so the fender trimming I had to do may not apply to everyone (and I didn't have to do it on the driver's side, so part of it is just body tolerances)
