Are you welding the pipes butted together or is one pipe expanded to fit over the other ? For the butt weld i would recommend the 3/32 rod and as low an amperage as you can and still get good penetration for the overlap i would use the 1/16 and high amp because you have material behind to fuse to (Personally I would tig this) also are you welding the pipe on the bench and then installing or do you have to fit together on the car and weld in position?