I believe a 0.5" spacer would work (though the widest I'm aware of are 7/16", which would probably do), but the question then becomes: is it safe to run a spacer that wide?
Again, from my research, its not a good idea... to do so would require getting longer wheel studs (0.5" longer), which should be readily available, but is not the purpose for which they were designed. Extra stress to the studs would be caused by the wheel being further offset from the hub because of the spacer. Not good. The better choice would still be an adapter like the ones on Kar Kraft's site. Obviously, the 1" offset of those spacers would take the functional backspacing from 4.95" to 3.95", which is no good.
Now, I assume you're still talking about using the Mach 1 wheels. The tough thing there is that the AFS versions are apparently only available in 17x9, and the OEM versions definitely have more backspacing than 4.95". Now, if the AFS versions were actually available in a 17x8" in 4.95" backspacing, and you use a 0.25" spacer (which is considered safe), that would give the wheels a functional backspacing of 4.70", which should work. Like I mentioned earlier, I've heard of several people using 8" wheels with a 4.75" backspacing. You migh search the archives for more info, as I recall there were a couple threads about it (mainly on 15x8s rather than 17x8s though).
Ultimately, all this is still just theoretical until you actually measure your own car. There are still enough variations in sheetmetal fit, rear axle centering, frontend alignment, etc., that measuring your car might be the best way to know exactly what will fit.
Just more to think about...
