The question has been asked often. From what I've read, it can take it as long as you don't make hard drag launches with sticky tires. Well, I plan on doing just that when I get my car built up. And if my T-5 breaks, I'll just upgrade to a G-Force.
Some people break them behind stock 5.0 HOs. Others lift the wheels doing mid-11s and hold together.
I wouldn't count on one lasting forever behind a 347 of any kind, but if it's just a hot street car without slick-equipped launches, then it will probably be fine for some time. The lighter the car, the easier it is on the trans. Early Mustangs are on average a few hundred pounds lighter than a 5.0 Fox3.
it is all in how you treat the trans. i have seen nwc T5's last several seasons in 9 second drag cars, and i have seen people break the good early top loaders with stock small blocks. if you are a ham handed driver, then the T5 is not for you, unless you get one from g-force or other T5 builder. if on the other hand you can drive cars with bad parts and never break them, then go for the T5.
one more thing to remember, the T5 has only so many powershifts in it, so when you break it, build a stronger one.
On a G-Force, it may be more cost effective to start shopping for Tremecs at that point. A G-Force gearset alone is nearly the cost of a 3550. The downside is that a Tremec will need a bigger trans tunnel depending on the car.
I have a similar setup (347 puttin gout 400 hp and 420 ft lbs) and decided to go with the T5Z which is rated at 330 ft lbs constant. Based on what I have read, the tourque ratings are based on constant stress test, so I was less concerned about the transmission failing since I am not drag racing and won't be abusing it other than the occasional burnout. I considered the stronger Tremec's, but too much fabirication involved to make them work properly. Like others have said, if I break it I will upgrade to G-force.
Bite the bullet and get the Tremec TKO 600. Its a STRONG tranny that will hold together. Furthermore, it fits without cutting up the firewall, just a different cross member. You can find them for around 1800 + 200 for the cross member. It'll be the best 2k you'll spend if you have that kind of power. Unique Performance tested these Trannys up to 1200 ft/lb of torque before they broke...
SuperSnake428: not a good idea on a 65/66 as the tranny is too broad for the tunnel. The tunnel must be raised about 2", otherwise a poor driveline angle will result.
Use an aftermarket shifter on a T-5(z) that has stops to restrict the throw - most T-5s are damaged by over-aggressive shifting that damages the forks, or powershifts that eventually trash the gears. Use a good shifter and don't powershift, and the tranny will last. Typically, you can't get enough tire under a 65/66 to hurt a T-5(z) if it's shifted properly. That is, the tranny is rated at 330 lb/ft, but typical tires will break loose before you can get that much torque to the ground.
Why anyone would want to upgrade a t-5 is beyond me. When the day is done you will still blow the case. Its like using a late model roller 5.0 block and bandaging it with a "girdle". Its not the bottom end that comes apart . . .
Aside from my opinion , the point has been made.
1. DON'T powershift it
2. DON'T dump the clutch at the strip with ET whatevers
3. DON'T force it into third
4. DON'T powershift it
a friend of mine has a 90 lx with a 347 that put down 301 rwhp and 360 rwtq with the t-5. No problems as of yet but its only seen 285/35/18 sumitomos.