How to reduce current
If the stepper motor only draws 350mA max, it will only ever consume that much (and probably only at high loads). The driver is capable of supplying 600mA, but that doesn't mean it's going to insist the motor uses it all!
There should be no overcurrent problems from the motor's perspective, and I would expect the driver IC to have in-built protection to prevent more than 600mA being drawn if the motor were to take leave of its senses and go crazy.

Any rudimentary current-limiting may just add unnecessary drain on the batteries, or have a detrimental effect on the circuit in general. The whole point of a motor controller is to eliminate these sorts of headaches