Correct T5 for a '68 Mustang

the V8 T5s are going to be the strongest ones available. they have either a 2.95 first gear o a 3.35 first gear. the 3.35 first gear is the most common. if you can afford it, the Z series T5 is the one to get.
 
Also, it has to be the same as the ones from a Fox-bodied Mustang ('93 and older) otherwise the shifter won't be in the right place. Make life easier and grab all the stuff (trans, bellhousing, flywheel, block plate) from the same car. You'll need a new crossmember and clutch system (buy a hydraulic with a slave cylinder), but your stock starter and driveshaft will work. Ideally, a late '80's Mustang GT is good since it's a WC T-5. If you swap to the newer flywheel, have your new flywheel balanced to match your 28oz imbalance motor. If you use your older 4 spd setup, you'll need to buy an adapter/spacer plate to mount the T5 to the older bellhousing. Don't forget the fulcrum spacer either or the clutch pedal won't have enough travel to disengage the clutch.