I have a 1990 5.0 mustang with an estimated 345 hp, and I have killed three T-5 trannys in two years. I was woundering if threre was a better tranny I can use with out many mods.  

AOD

Just kidding.  How about a Tremec 3550.  You would need a different bellhousing and might need to cut the welds on the tranny xmember, but other than that it is a direct replacement if you use the tremec shifter.  They cost a few hundred more than a regular t5.

Tom

The tremec 3550 is only good for about 330 lb ft of torque.

A good tranny...but can be busted. If youre gonna be hooking up good with street gears than Id look maybe to a tko500...theyre more pricy but well worth it...also...might shop ebay for a used one. Id love to have one but thats just not in the budget for me..."cries a solitary tear". You can look for a used t56 but finding the adapter bellhousings and then fabing the crossmember will cost you more than the tko.

i cant think of many stock manual trannys that are good for more than 340 lb ft...at least without shedding the overdrive.
If you wanted to grab a 4 speed than you can get rebuilt t-10 for like $500....still a touch salty in my opinion...but people are going that route.

Id rather row my own gears and break 10 trannys than resort to slug shifting a 4ton slushbox....but people have called me nuts before.

Sorry to rain on the parade.
-Shred

For a direct replacement Heavy Duty trans. Check out G-force at http://www.g-forcetransmissions.com/tran_gt-5.asp 

I use there, bigger, GF5R behind my blown SBC. Its making 800 HP.  Bill.

The 3550 is pretty stout.  I paid extra and got the TKO since I needed a clutch disk too.  The regular TKO is the same as a 3550 except his has the GM splined input shaft and a 31 spline output shaft.  Maybe ask some of the shops that sell these transmissions like D&D or Hanlon.  I got mine on a group purchase on the corral a few years ago.

Tom

The Tremec TKO600, I installed one in a 03 Mach 1 earlier this year. It has a vortech and numerous other mods, and the owner is soooo happy with it. It's pretty much a bolt in.