As you mention 321 is stabilized with titanium. The titanium has a higher affinity for carbon to mitigate sensitization. Other than that, the A number is not affected, and the filler metal F number is F6 for GTAW, F5 for the SMAW.

From a code standpoint, your WPS is qualified since the group number is only applicable when notch toughness is a requirement. That being said, your WPS will have to be revised to include the combination you wish to weld. As a personal observation; it isn't sufficient to simply say the base metals qualified are "All P8 to P8" and list all "F6" for the filler metal. Most welders have no idea what base metals are included as P8 and are clueless to what filler metals are included as F6, A1. So, it is best to be specific as to what base metal and filler metal combinations you want. One way to provide the information to your welders is to develop a matrix of the specific alloys of base metals and recommended filler metal combinations.

This subject is covered by the seminar titled "The Atlas of Welding Procedure Specifications". You can find more information on the seminar under the subject heading "Training and Education".

Best regards - Al