The correct stuff the Model A parts houses sell is the best emergency packing to use. I installed the lead packing rings WITH grease around them about 6 years ago and my pump hasn't leaked a drop since. If it did start a small leak, I'd just screw the brass packing nut up a bit. It doesn't cost much to buy the lead packing rings and carry as a spare part if you feel the need to have something for an emergency.
If you're rebuilding the pump, it's best to buy the rear bushing with the double lip seal. I didn't do that when I rebuilt mine 6 years ago, but will change it someday when I have the pump off, and will use the sealed rear bushing on all future rebuilds.