The backspacing is different, they will be VERY tucked into the fenders and the might hit on your suspension without the spacers. You are going to be $100 into spacers minimum, probably more if you do it right and get the ARP hardware to run them correctly.
Honestly, if you want new wheels then hit up CL and multiple forum classifieds and see if anything is for sale near you for your year which would be anything for 94-04. Or just buy new from AM or LRS or wherever.
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 ****.