15 inch 4 " (SBP) steel wheels ?

it adds 1" per side. so if you're currently running typical old mopar offset (+1/4"), to keep the tire in the same place, you need to run around +1.25" offset. '94-04 mustangs are +30mm offset (1.18")...if you like the 17" look, Nitto 255/50R17's on mustang rims would work great....without rolling the 1" wide fender lip on my duster, I can fit 235's on stock rims.

if you want to keep the older low offset wheels, your best bet investment wise is to do a ford explorer 8.8 with the driver's side shortened using a 2nd passenger side axle, that ends up being like 56.75" wide (3/8" narrower than my stock 7.25" axle). can ususally find 3.73 limited slips for a couple hundred bucks, maybe another 50 for the 2nd passenger axle shaft, then time and a few bucks to cut down, weld, and put new spring perches on....