If you go over the standard opening sizes - you will have to recalculate the flange in accordance with ASME VIII, because you will be out of the limits of B16.5
You should go to UG-36(c)(3): the upper limit for openings not requiring reinforcement is 2 3/8 in. for the finished (corroded) opening diameter.
Of course this will make a big hole in a 3" flange, but if you consider that the blind thickness is the same as the thickness of the flange... prex
Check out Table 7 in ASME B16.5. This table is titled Reducing Threaded and Slip-On Flanges for Classes 150 to 2500. The table gives the smallest size of reducing outlet from the "blind" flange that requires a hub for each pipe size through 24" NPS. Threaded and slip-on connections are both covered. If you want to have some other kind of connection or have a flange larger than 24" NPS you will probably need to calculate per ASME VIII-1 (UG-39, Appendix 2, & Appendix 14).
This links to an article on the Hartford Inspection website, written by Tom Pastor. Although these aren't the official words of the Code Committee I think we can take Mr. Pastor's words to the bank.
