standard non-deep dish 17x10.5...
 

will 315s fit 99+ w/o problems similar to the deep dishes???

if i lower my mustang, do i really need caster/camber plates and/or bumpsteer kit???

17x10.5 will fit the 315 tire and depending how low you go with a drop will determine if you need the plates. up to 1.5 you dont need them but i would still get a wheel aliement.