I also tried to expand a 1.75" .035" 321 stainless pipe today.  I need to
make it fit the 2-1/8" hole in the exhaust flange, and there's no way I'm
going to be able to do this.  It seems like a lot to stretch a thin wall
pipe, and I was doing this as a test to see if it would work.
Unfortunately, I just can't generate enough force with the tailpipe expander
to do much with it.  I used an impact wrench, with about 120 psi of
pressure, and the most I could stretch it was maybe 1/16".   I assume a
custom exhaust shop could do this, but would the 1-3/4" pipe expand
successfully to be 2-1/8", or would it split?  I'll either have to have it
stretched, or buy something pre-stretched, or perhaps make a new exhaust
flange.   Ideas are welcome.