A couple of comments:
1. My understanding is that the 650b tire width is 47mm - this is the width of the WTB Horizon Road Plus tire that came on my Current purchased in 2020.
2. 2.8" would likely be much too large. I believe ~ 2.0" is about the max tire width that would fit with fenders and perhaps slightly larger without fender. The specific tire being considered would also have an impact on this.