Why does the B31.1 limit S/O flanges to NPS 4 and Class 300 per Par(s) 104.5.1(A), 122.1.1 (F) and Fig 127.4.4(B)? Can or should one design around this restriction?
The main reason should be the lower fatigue resistance of those flanges with respect to weld neck.
If you are bound to B31.1 as the code, I can't see how you could circumvent that rule: you should switch, if you can, to a less demanding code, such as B31.3.