Aftermarket wheels will fit. If you happen to have a big front brake kit, like Brembo or Wilwood, they require 17-inch or larger wheel with certain spoke shape and wheel offset. For SN95 mustang, a 9-inch-wide rim with a 6-inch backspace works well.
If you get a 9-inch-wide wheel with zero-inch offset, it means that the location of the wheel mounting surface is going to have 4.5 inches of rim width on each side of the wheel mounting surface. If you have positive offset, it would move the rim closer to the centerline of the chassis. The distance between the wheel mounting surface and the back side of the rim, in inches, is called backspacing. Therefore, a recommended 9-inch rim with 6-inch backspace is the same as 9-inch rim with a 1.5 inch of positive offset.
For your year of mustang, 1999 v6, you have your OEM wheel 15 x 7 with 5 inch of backspacing, or 1.5 inches of positive offset, if I did the math right.
Hope it makes sense. Whether you need spacers or not, you can decide for yourself. Once you have chosen the right wheel, you might want to ask people around what tires work great in combination with it.
Hope it helps.
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