need help! t5 or tremec?

It really depends. Some T5's grenade pretty quick under lots of power and some seem darn bulletproof. To be on the safe side I would recommend a transmission upgrade.

The TKO is a Tremec, like the Mustang is a Ford.

I would suggest buying the transmission from a Tremec authorized dealer in case anything were to be wrong with the transmission. If you are unsure if a dealer is authorized or not I believe you can find a list through the Tremec web site.

You can find the other necessary parts for a TKO conversion through various web sites/vendors. You'll need a bell housing, a fox body (87-93) clutch fork, a 31 spline slip yoke, GM syncromesh fluid (3 qts), a 26 spline clutch, and a dust shield for the bellhousing (again, fox). A new flywheel, driveshaft, pilot bearing, throw out bearing, adjustable clutch setup, short(er) throw shifter, transmission mount etc. are all just icing on the cake. Note, if you get the Ford Racing driveshaft for the 96+ cars it comes with a 31 spline slip yoke and fits our cars. I also had trouble getting a dust shield to fit with my MAC long tube headers, but one can be made from sheet metal with relative ease.

There are all sorts of threads about converting to a TKO, but I'll help in any other information I can give.

Unfortunately I don't hold any information about the 3550, maybe someone else can help there.

Also, you don't have to buy a new driveshaft with a TKO, but if you didn't already have an aluminum one it would be a good time to do it.

I don't believe you can buy the 3550 anymore. Not that you'd want to anyway, it's really not much stronger than the T5 anyway and you'd probably hurt it with the 347. If you want to stay with a T5, look into getting one of the G-Force or Astro Performance versions that are rated for higher HP & Torque levels than a regular World Class T5.