What kind of gearing on the belts???
I'm not sure of the exact motor you have, but is it wired bi-polar parallel??
The 201 could go up to 80V, higher is better for high speed torque.
I'd assume @ 900 Oz its a nema 34 motor??? smaller is sometimes better for high speed torque... the higher inductance and higher rotational mass can slow you down...
6tpi at 71 ipm = 450 RPM * 200PPR * 10 microstep = 15000 pulses per second * your gearing???...
I run R&P on both sides of my X ... nema 23 - 387 Oz keling on a G540 ~ 36V and have achieved over 1800IPM gearing is only just over 2000 microsteps per inch.
That is basically one turn of the motor per inch.
With your big motor you could try to gear up your final.. lower motor speed should = more usable torque. But as soon as you spin your ACME to fast it will whip and cause you problems... how long is the axis??
Everything I read leaned towards rack and pinion/belt/chain drive for high speed - plasma and laser applications... and screws for lower speed higher torque - router and milling application... you can also get multi-start thread and high ration ballscrews, but I would think the plasma dust could gum them up real fast if not sealed... Rack and pinion with the rack facing down is decent design.