New wheels

I would be more concerned about the bolt pattern. I believe the mustang rims are in milometers.
Yes they will fit,but are not the correct 4.5" pattern.

This is not true.

[FONT=Arial,Helvetica,sans-serif]FORD[/FONT] [FONT=Arial,Helvetica,sans-serif]MUSTANG GT (OPT.), COBRA[/FONT] [FONT=Arial,Helvetica,sans-serif]1997 >[/FONT] [FONT=Arial,Helvetica,sans-serif] Metric: 5 X 114.3
Standard: 5 X 4.5
[/FONT]
4.5" = 114.3mm