Is it too late or will we just need to make an appointment to get an I-551 stamp if the NOA doesn't come in time?

It is not too late to file, as long as you file before the greencard expires you are ok... as for getting the I-551 stamp, it is unlikley that you will get it... most local offices will only give the I-551 stamp if your NOA1 extension letter is about to expire... remember that it is just the card that will expire not your status... for work you just need to show an unrestricted SSN and state ID/License.... for international travel you will need to wait for the NOA1....

Kez

I've read posts of some VJers who were able to get a I-551 stamp when their extension letters did not arrive before the GC expired. You'll have to show proof that you have filed your I-751 (proof of delivery or biometrics letter).