Folks have had similar issues on a number of (mostly Japanese, I think) cars with those DIN brackets and have worked around it in a number of ways:

- bending the brackets a bit
- widening the horizontal axis of the scew holes on the brackets
- drilling holes in the sled/brackets so the sled can be attached to the brackets with small screws/bolts

If you search Installation for combinations Subaru/WRX/Impreza, you'll find at least 3 examples from Brad (SE_Sport_Driver), myself, and most recently Nick, that may help. I think there have also been some double-DIN Honda installs with similar issues.

I don't know that there's any pat answer as to what you need to do, but I'd guess with judicious application of drills, files and such, it'll work.

Hope that helps a little!