Shouldn't be any problem at all to stitch weld a muffler with the mig welder. 

I've stitch welded down to 20 guage with .035 wire.  It's almost a necessity to have an auto-darkening helmet though, since you will be 'stitching' for less than a second and cooling for about a second -- or a bit more, and then repeating.

Oxy-acetylene welding is good too, but remember that flame stays hot no matter where you point it, and if you're not particularlly careful you can make other things hot that shouldn't be.  At least with the mig, when you release the trigger, heat stops.