Just called Master Hitch in Houston, TX (713) 974-4440 and the guy answering the phone admitted to doing the work.
The gentleman on the phone said he used the Curt PN 11033. http://www.curtmfg.com/part/11033#tab=vehicles
So I guess I was wrong in guessing it was the 11771???
The 11033 fits the 2007-2010 328xi and 335xi.... Which makes me wonder. I doubt those are the wagon styles.
The guy told me that the bumper part fits up fine, but the center bar that extends to the frame cross-member (by the axle) is too short and needs to be modified.
Modifications are: After mounting the main hitch to the bumper, note how the center bar is too short. Grind welds off of center bar where mount plate is (axle side). Torch / heat up center bar to straighten out as needed, then re-weld the plate back on to the center bar aligned with the available holes in the frame cross-member.
Seems like I don't have the tools for this job. Maybe I can do half the job then have a local shop complete it.
-----> Can the original poster please confirm the actual Curt PN installed on your vehicle?