SA351 covers cast stainless steels anywhere from plain vanilla austenitic stainless, to superaustenitic to duplex, and can be classified as P#8, P#10H, & P#45.
ASTM A297 deals with heat resitant Fe/Cr and Fe/Cr/Ni alloys, but as far as I can find, it doesn't have the status of an ASME specification, so this material might just be unclassified in Section IX. There are also quite a few grades in that specification.
Also, UNS numbers always start with a letter followed by 5 numbers, but the A297 I have does not list USN Numbers anyway, so that wouldn't help me much.
However, if you are going to qualify to ASME IX, you need to have a procedure with the same P# as the SA351 alloy welded to the unclassified A297 alloy for starters. I wouldn't think impact variables would apply since you are working on furnace materials. If you are working to a construction code (like ASME section VIII), you also need to address that to see if there are any other variables you need to use during your qualification, such as PWHT, thickness limitations, etc.