A rule of thumb for welding disimilar metals is use the filler wire from the higher quality metal. In this case use 308 filler (tig) or wire (mig).
The best results would be with inconel filler wire RN-82. But it is unlikely you will find a muffler or exhaust shop that would have it. The stainless filler will do.
Mike
------------------
Live near St.Louis?
Check out Mustang Muscle and Dyno in High Ridge, MO
Check out my
ride.