The spec for the NEMA 42 motor is either wrong or you are misinterpreting it wrongly. It would be unlikely to be able to produce much more than 20n/m and that would be a holding torque, as soon as the motor starts turning the torque will drop rapidly and in general the larger the motor the faster the torque drops. NEMA 34 motors are largest practical size for stepper motors I have used NEMA 42 and they do not perform well. Motor torque for small motors is usually rated in Newton/Meters or CMs not KGs, A Newton is roughly 100 grammes. (0.981 Kg) You may need to redo your sums.