It is indeed a 320x. We purchased an 8mm encoder from US Digital (
http://www.usdigital.com/products/en...rotary/kit/e7p) which fits perfectly on the shaft. We're using an old 24v power supply that was laying around which can source at least as much current as max the 320x can supply to the motor, which is 20 amps. I probably should have spent more time looking for a more suitable motor (higher voltage, the 320x can take up to 80v but the CIM can draw upwards of 100 amps, so it's operating at a relatively low torque), but $25 is a heck of a deal! I'll be mounting it today and hopefully will have at least one axis working. The motor shouldn't hit the driver's current limit if the gantry is accelerated slowly enough, but the hacked firmware we're using (
http://reprap.org/wiki/Sjfw) supports adjustable accelerations on each axis.
The current motors only have a holding torque of 250 oz-in, which decreases sharply with speed, and, as I've had the opportunity to learn, steppers need to be accelerated very slowly to reach the speeds we need (at least on such a large scale). Currently they're being accelerated at 30mm/s/s up to about 10cm/s, but we'd like to at least double the maximum speed and reduce the amount of accelerating travel.