The forging vendor can most likely provide you with the SIFs. But as in most cases with High Pressure heavy wall fittings, the SIFs will most likely be close to 1.0. Review the nominal stesses at the center point and see how much margin you have.

If you have stresses in the 80% plus range (SIF=1.0) then you probably have huge support loads that cannot be designed in the first place.