The Perrin damper lockdown is not a U-joint. It is a modification to a U-joint from an Outback, Legacy, or WRX. The Perrin part does not change the form factor / length of the U-joint. It's only to increase the rotational stiffness.
If you are swapping this rack into an otherwise-unmodified Outback, I'm pretty sure you need to use the Outback U-joint, as that's the only one that's long enough. I believe Legacy, WRX, and STI U-joints are all the same length, which is shorter than the Outback's.