They look nice motors. For the size of your machine (8*4) I think you will be underpowered. I am also building an 8 * 4.. (see Project log... New Obsession) and I am also using a Belt Drive for the X Axis.
I will be using Servos (880 oz.in) geared down 9:1. on the X.. Even with this size I am a little concerned I may need a larger motor for the X Belt Drive. Gearing should provide about 3HP at the belt.
The motors you are looking at are Steppers which have their torque at low RPM's. This may mean you would be looking at no gearing to obtain enough torque.
Your gantry is not too heavy and depending on your bearing system will determine how much power is needed to drive it.
If you are going to use steppers I would recommend using a stepper over the 1,000 oz.in for the X drive. Even for your Y axis you would be wise to find something more powerful...