I would ensure you get a solid answer before you go ahead with these motors.
I also am building a steel 3 axis ( cutting footprint 800 * 1200 * 200) and I was recommended minimum 400oz/in stepper by this forum.
Since then I have gained an impression that the advice was good.
My gantry is quite heavy and missed steps would cause me a constant headache.
I am not an engineer and so I hope my feedback does not mis-lead.