Quicker and Safer
I'm going to go with Master Plumber Mark over srdenny at this point. Of course, I already have everything back together, including the stack. I'll be checking for leaks very shortly. But after spending two days trying to fit a PVC 45 into that old bell, quick was alreadly a lost option. Also, given that the majority of the main is 105-yr-old iron, I'm not even testing for air-tightness. So safety isn't really a concern (and I already used a MAPP torch for 30 minutes to free up that bell).
Just to recap, for anyone else reading this thread, here are possible ways of transitioning from DWV PVC to an old iron bell:
Use a Fernco or Tyseal "donut" type O-gasket lubed liberally with liquid soap (or maybe shortening - I'm not sure about this). Use a sander or file to taper the end of the PVC fitting or pipe.
If, as for me, this doesn't work for you because the fitting's just too tight, you're best to use either a PVC Soil to DWV adapter (with a flare on one end and a PVC bell on the other) or an iron fitting or piece of pipe to create a new stub. If you use iron, the new stub will connect to PVC with a rubber no-hub coupling. If you use the PVC adapter, obviously your next connection would be cemented.
One other option offered to me, which also didn't work, was to use a 5"-to-4" rubber Mission (no-hub) coupling, with one end on the bell and one end on my street 45. This would probably work on a straght tee or a lone iron bell, but a bell on the arm (angled inlet) of an old iron wye is too close to the bell of the straight inlet, making it impossible to get the street end of the PVC 45 down to the bottom of the bell and keeping the 5" side of the Mission coupling from sitting fully over the bell.
If you're using a PVC Soil to DWV adapter, theoretically you can pack this with layers of oakum and plastic lead, epoxy putty, or a final layer of marine-grade silicone. Using all silicone, as some have suggested elsewhere, would result in a loose joint.
If you're using an iron fitting or pipe stub, you can pack it as above or get a plumber to do it with molten lead. I suppose if you're really enterprising, as I was almost tempted to be, you could buy lead ingots, a pot, etc, and melt your own. I'm not advocating this, however. As Mark and srdenny have pointed out, there are also lead alternatives in the form of a rope/string or a wool, which would get packed down with the oakum.
Finally, a few people told me I could directly pack a PVC fitting into the bell with one of the methods above. I was wary of this advice. When the supply guy finally told me to use iron in the bell for the best joint (leaded or otherwise), I felt like this was the best advice. The problem using anything other than a purpose-made rubber gasket for transitioning from an iron bell to PVC is that packing materials weren't really designed for plastic. Or at least that's my interpretation. I read somewhere that molten lead could even be used on a transition to PVC, but that sounds scary to me.
I hope this helps someone else and keeps them from spending two solid days repairing their main or stack.