BF Goodrich and Nitto also make Drag Radials that will fit your factory rims. Getting into the 12's is going to take more mods than what you have now. This link will show the times of 2 Valve N/A Mustangs from the 10's on up thru 15's. Some have member pages you can view and see what they have done to get these times:
http://forums.modulardepot.com/bestets.php?filter=2VNA
http://forums.modulardepot.com/bestets.php?filter=2VNA