Actually the Vette uses it too in addition to the new GTO. Small variations between all the applications though. Check
www.sixspeedinc.com. They have a great reputation on LS1tech.com. My 99 TA uses the T-56 and is big factor in its performance in mileage, acceleration and ridiculous top speed.
Should run you about 1500 without a core I believe. Not sure what else is involved for the late model Mustangs, but like I said the Cobra ran it so it should be a bolt in with the right parts. Check the Ford boards out to see.