17 inch rims are prob your best bet, the amount of weight you save between 18 inch and 17 inch is enough to label them performance, and 18's appearance.
Shiftkits I wouldn't even bother with. If your mechanically inclined enough to install your own then your mechanically inclined enough to do the Jerry Mod. It is cheaper and apparently a little better. www.tccoa.com
is the link for it. Goto their techs and look in the tranny section for JMod.
If you want to buy a shift kit anyway then i would go with B&M or Transgo. I think Hayden makes one too but that isnt a definite. You can get them from A&A, Jegs, Ebay, and some mustang sites around. Look for anything for a 4r70w or AODE trannys.