The minimum size list in MSS-SP-44 is 12" NPS. This material is basically for High-Pressure Transmission Piping. I think the dimensions for the flange sizes that overlap B16.5 match and the pressure ratings are the close if not the same.
I'm stepping out of my area of expertise, but I have to think that you want to match your flange tensil strength to that of your pipe so you don't have a problem requiring a significantly heavier wall at your flange hub end than that of the wall thickness of your pipe.
Your temperature is covered, flange class covered, material covered but not the sizes that you are using. I think you need to do some comparsions of materials, ratings and flange dimensions between MSS-SP-44 and B16.5 and make some sound engineering decisions or calculations.