99-04 Mustang with Solid Rear axle (V6 and GT)
3 inch frontspacing seems to fit fine, can go a bit around that but about 3 inches works good. The LOWER the car is the more the wheel tucks in due to the geometry of the front suspension FYI. As the car is lowered the A Arm piviot up and inward so the wheel will sit in more. Backspacing I would keep it NO more than 6.1 inches. Even then it may rub the sway bar, 03/04 cobras had this setup and used rack limiters.
I cant tell you on a Solid Rear Axle, but for an IRS 03/04 cobra front spacing 3.5 works but 3.75 MAY require fenders to be rolled, 4.0 will (do not go more than 4.0, 3.75 looks best to me). For backspacing depends on if you have quad shock but most 10 or 10.5 rims are fine with a frontspacing around 3.5-3.75 inches
offset is the mounting surface location inside the wheel. Think of it like this.
If you have a wheel thats 10 inches wide a zero offset would be that the mounting location is exactly half way of the wheel or 5 inches. If you measured the amount of the rim that is in front and the amount that is behind of the mounting surface they are both 5 inches.
Most measure offset in MM's instead of Inches 25.4mm = 1 inch
Positive vs Negative offset is the ahead/behind the center line of the rim the mounting location is. For mustangs the offset should be so that more of the rim is inside the mounting location than outside.
Backspacing is the amount of the rim behind the mounting surface inside
Frontspacing is the amount of the rim outside in front of the mounting surface