1. The steppers I have have too high an inductance and/or are not powerful enough to perform as the original Superior Electric steppers did (even though their inductance was much higher and their power (on paper at least) was lower.

2. Steppers are probably not going to perform well enough for an industrial machine like this - they're just not going to be smooth enough for the type of fine work this machine is capable of. The latest version of this machine is fitted with servos and I think I'll eventually have to go down this route. Unfortunately, I know less about servos than I do about steppers.
How about a pic of the machine or a link model etc.