The 3550 edges the T5z in torque cap.
The 3550 has a 1st of 3.27 vs 2.95. It also has a .68 OD vs .63.
The tremecs will require an aftermarket bell housing(or an early 80's stang SROD bell housing). I think to T5z is bolt in.
The 3550 is slightly better at the track due to the short first. The t5-z is more street friendly and Od is deeper.
The T5z shifts nicer(so I've heard). I have a 3550 and I like it.
If you are going to run alot of boost I would recommend the TKO. It will handle almost anything you through at it. You will need the bell housing, a different clutch,new yolk on the driveshaft(think it comes with the tranny).
Also the crossmember might need mods.(grind old welds and slide into position and re-tach)
I think that's it but others will add.
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82 Mercury Capri 302.
Comp. Cams
Tremec 3550
3.73's
12.8@106 N/A
Kevin's Capri