I know this thread is a few years old, but I just finished installing mine and had the same problem, so I figured this info could be helpful to others.
Essentially the fwdperformance girdle's bend is too 'round' and hits the bearing retainer. I simply used the grinder and grinded the girdle where the retainer would rub. Grind a bit, apply some primer, see where it rubs again, grind more. Repeat until the girdle sits flush over the diff case - this is important to avoid leaks.
Also, most of the bolts that were packaged with my girdle weren't the right bolts. All the bolts are M8-1.25. The bearing retainer used 30mm bolts with a small washer (the 35mm ones that came with the kit were too long for a few of the holes) and the diff case used either 25mm or 30mm.
As a side note, the shift linkage bracket is spaced using three nuts screwed to the studs. If you don't feel like readjusting your shifter linkage, simply mount the bracket directly to the girdle. The girdle is about the same thickness as those nuts -- this way the bracket is no further back than it was stock (and gawd knows there's not lots of room to work with already).
Under load the tranny just feels more solid with the big girdle holding things up. Maybe it's just me.