the cobra's actually just got the standard 3.35 wc t5 that all the other mustangs got. The difference is that the cobra's got the tapered bearing on the front, where the other mustangs did not. This tapered bearing is shared with the z specs.
A new z spec costs $1300 + ship.
You can build your own z spec for $400 or less, give or take for the hard parts which includes 1,2,3,4 gear, new cluster, new input...+ $150 for new syncro's, bearings and small parts and you've got a z at half the cost.
the z spec was a ford racing only trans. The easiest way to tell if you have a z is the 2.95 first gear ratio...rotate the input, count the output turns..all other w/c trannies received a 3.35 first gear ratio. Also, don't confuse a z spec for an 83/84 non world class t5. I see people get screwed this way. the 83/84 non world class t5's got 2.95 first gear ratio but are NOT world class spec! You can always tell a nwc t5 by the bolt on the top middle pass side of the case. If it's a hex bolt, it's a nwc t5. If it's a torx bit, it's a wc t5
a z spec will handle up to 450 horse, but as mentioned if you're on slicks every weekend, look to move to a gforce or tremec....for a mostly street machine though, you'll be just fine. The thing that kills t5's is powershifting, period. My friend launched a 321 at the wheels coupe for 2 seasons on slicks with a z spec and didn't break it. I built a t5 for a friend and within a month he shattered it by powershifting at the track with nitrous. don't powershift!