880 oz-in onto a 1/4" pitch screw would give you a theoretical 1380 pounds force on the nuts, that should move it
More importantly, try and find a graph of torque against speed for the motors so you know how many volts you will need to get a credible rapid transit on a G0. A smaller motor might hold torque better at higher revs and still move it okay.
Microstepping is springy, use it to smooth the motor rather than to improve resolution.
A NEMA34 direct drive will rise above the top of the bed. Might you ever want to machine anything that overhangs the bed?
Only XY control? Sounds like you want to drill rather than mill?