Were the ones you got from Summit a completly different design, or did they simply not fit over the output shaft and bearing? If that is the case, I would be willing to bet you have a 2.3l T5 in your car.
I ran into that when I bought my car, it had a T5 out of a 2.3. The retainers I bought wouldn't fit, so I measured the diameter of the output shaft and found out it was not the correct size for a 5.0. I went to the local boneyard and pulled a T5 from a GT...only to get it to the shop and find out that IT was also a T5 originally out of a 2.3. So there are 5.0 Mustangs out there with 2.3 T5s in them. The only difference is the pilot bearing, which you will need to replace if you get the correct T5.
In other words, you can either find a retainer from a 2.3 T5, or go get a T5 made for the 5.0 and a new pilot bearing.....IF that is the problem you have.