you can use mild steel wire to weld stainless and vice verser, yoll need to turn the power up more than normal and the quality of the weld will be no where near as good as when you use the correct wire on the correct material.
if your exhaust is standard im sure it will be mild steel and if the new stainless peco back box is correct for your car it should just clamp on in place of the standard rear box, so you should only need mild steel pipe to replace the silencer, and with it being made from mild steel you should be ok unless its rusty/rotten in which case it aint worth it. Pending on the age of you car might be worth removing the 'cat' as well whilst your under there to free up some extra hp!!
