if this is what your looking at
http://www.centroidcnc.com/centroid_...controller.pdf
the step & direction signals from centroidcnc controller needs to be connected to 3 stepper drivers that need to match the requirements of the stepper motors
are you going to re-use the existing stepper motors or replace them ??
( higher the inductance of the motors the higher the voltage needed to step the motor at a high speed )
with no details of signals the Denford cnc spindle control needs you may need to replace the spindle speed control
if it can not use the analogue 0 to 10V DC control voltage from the centroidcnc controller
John