vwjoegulf,
Use a 347 filler metal.
There are probably alternatives that others may mention but 347 is the first choice for welding 321.
Hope that helps,
Cheers,
Shane

There is a 321 filler metal, but it is in short demand since 347 is normally used. If memory serves me correctly, 321 (Ti added as the stabilizer) can only be used in the gas shielded process.It is not recommended as a sub-arc filler since the Ti will not fully transfer across the weld arc. I think 321 is pretty much used in the aircraft industry.