Wheels can be repaired using a special epoxy (whose name escapes me but has a "7" in it.)
Basically what you are doing is widening the cracks slightly in order to get the epoxy into it. File/sand the repair when dry then primer/repaint the appropriate color. If you have a woodgrain wheel, there is special color paints for that.
Poke around on the internet as I am sure you will find an article. Most Mustang magazines (and I think Eastwood) have had articles on how to do this.
Personnally, I think that it would be better to get an all new wheel from a Mustang parts vendor (if your color is available.) Painted wheels are normally less durable and will need touched up periodically.