hmmmm......yep even the catalog page I pointed you to above says the headstock and tailstock are both MT2.
So either,
-Southbend started making them different over the years, or
-a previous owner changed the spindle, perhaps to get a bigger thru-hole.
The headstock may be an MT-3.
Do you have another Morse-taper arbor in a drill press that you know the size and can try it in the lathe?
A drill press may have a MT arbor even if you don't realize it.
See:
http://www.hobby-machinist.com/threads/griz-g7946-drill-press-runout.34761/#post-294094
-brino