I-551 stamp at another office is certainly possible
I know lots of folks who did it. I personally did it myself. My I-485 approval notice instructed me to go to downtown LA, but I got mine stamped at Santa Ana. I got plastic card within 3 months from the stamp. Sometimes under exceptional situattions, few INS offices do not take you in. You have to tell them a reason as to why you are showing up in an office other than the one on the notice. Based on your situation, think of a good reason.
It doesn't hurt though to go and get it stamped at the original location. I did it because I thought Santa Ana would be a more convenient location compared to downtown LA. This was purely based on the stamping exeperiences I read on this board.