I'm not sure about the 05 Legacy, but if it's like the 02-06 WRX, check the link in my signature for some install pics. It seems like a pain in the butt, but all it really takes is notching out a TINY amount of plastic in the bezel to make it fit. You'll still be able to toss in your stock radio when you sell the car if you want. My buddy just did.

Even if you had to buy a new 90 dollar bezel, it's cheaper than buying a new headunit.

EDIT: Link: http://persiconi.com/wrx/images/audioinstall/install.html