17x8s are what came on your car originally. Most wide rear 18s are 18x10 although AFS makes them in 18x10.5 sizes. Spacers are fine but for looks and you ONLY want to ever get hubcentric ones. The $50 ones from AM are NOT hubcentric and will cause vibration issues.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.