My experience has been that in cases where a wheel you want to purchase does not come in the exact backspce or offset you're looking for, the use of a spacer is acceptable. I have also seen spacers used when somebody wants to push out the rear wheels a bit, usually to clear some wider tires on the stock rims. In all cases, it is STRONLGY recommended to use a hubcentric type spacer. In the Mustang circles, these hubcentric spacers have not caused vibrations... I would try and avoid going over 1/2" thick. Get a good quality spacer, usually made from CNC machined lightweight aluminum... they either come with or without integral studs... if you get those without, you may need longer studs to accomodate the spacer thickness.
