Mild steel exhaust tubing works nicely for stretching, but stainless (which already starts out pretty thin for exhaust systems) may have a tendency to tear/ crack when you expand it. I tried several times:-/
LDH has a good looking reducer up there, I wonder if it could be had at a good price in a short time? I ended up finding an extra piece of tube, cutting about 1-1/2" for a sleeve to slide in my muffler, and cut a slice in the side until it fit snugly. Added a few tac welds to hold the sleeve in place, shot it with some BBQ grill paint, and have not had an issue for 6-7 years.
30,000 mile 919 survivor. No plans of stopping the abuse any time soon.