The stock differential is very weak and I've seen first hand several that have blown up. One of my buddies who used to be on here quite a bit bought his SVT with a blown differential years ago. The white Cougar I swapped the motor and rebuilt the trans in had a differential that was damn near about to blow as well and that was a completely stock 2.5L with only 80k. But, when I installed my torsen in my SVT the differential looked like new. It's all in how you drive the car, if you're going to hammer on it or drive it hard, expect the differential to blow if you don't upgrade it. You've gotta pay to play!
On a side note, holy crap did this thread turn into a big pissing contest! Everyone does their builds or swaps for different reasons and each person needs to do the research about the types of swaps and determine how they want to do their build. I have my opinion on which swap is better and someone else has different goals for their swap or build and therefore has a different idea of what they want to do. There's nothing wrong with that.
Yea, the pissing contest something more than I expected, but I should have known better about. People tend to get real personal with their cars and quite often feel the need to defend any decision / part / project that they did. I know I'd be tempted to do the same if there was a debate of roots-type vs centrifugal blowers.
In regards to the differential, I'm not sure if I posted it or not, but I do fully intend to upgrade to either a Torsen or Quaiffe -- so the extra power will be compensated for.