Nad27 to nad83 ncat

I have a job just handed to me a rush the sky is falling job. All control is in NAD27 its a slam dunk for gps rtk so i formatted a csv file and uploaded it to ngs ncat. Converted to nad83 2011. I will have the crew set up on monument nad83 values. And convert everything back to nad27 when done. Anyway i did a test in TBC took the nad27 coords and changed to nad83 all northings matched ngs conversion all eastings were off a lot. Any ideas.