A carpenter for metal work? That's like calling a maid to cook for ya!!! Lol
A carpenter for their skill with a sawzall. FYI a super skilled carpenter is called a "mechanic" in the trades. They are called this cause their skill set allows them to do dam near anything. Metal work, fabrication, welding, etc. etc. I have known "mechanics" who's knowledge and skills have amazed. Its literally like," Is their anything you can't do"?
But back to the downpipe..The key to getting the oem downpipe out easily is to use a long enough blade. if the tip falls short of the total diameter of the tube that is being cut it turns into a Bludgeon Festival! The teeth on a sawzall blade cut on the pull stroke so make sure that the heel plate stays in contact with the material being cut. It will cut thru that mild steel like a hot knife thru butter. Also if you make the cut from the top to bot instead of vice versa the material will be less apt to bind because gravity pulls the halves away from each other instead of falling together and binding.