Converting a Bridgeport Clone

It has been a long time since I posted on this group and I notice there is not so much activity these days. Anyway on to my question:

I have a Bridgeport clone and want to fit a stepper to the knee and I am unsure of the size of stepper motor to do the job properly. I was thinking of buying the motors from Arc Euro Trade here in the UK along with their drivers:

http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors/Stepper-Motors

If it takes 11.5 foot lbs of force to lift the knee and there are 135.581795 newtons centimetre to one foot/pound, and I am thinking of a 6:1 reduction ratio on a toothed belt, what size motor would I need for reliable operation? I am confused about holding torque and moving force.

Would I need the stepper 160-010-00500 @ 4.2A 650Ncm or will 160-010-00450 @ 4.2A 350Ncm do the job?

Regards, Terry