I have a CNC'd chinese 7x12.
A Nema 42 is mega overkill for a 12x36 lathe. You probably only need a Nema 23 on X and a Nema 34 on Z. You can probably sell those NEMA 42's on Ebay and earn enough to buy some nice motors for the Lathe. If you sell the DRO as well you could probably buy two gecko's to drive them.
You should be running at the maximum voltage this board allows (+35V), even so you won't get close to the motors true potential.
Hook it up bipolar to a Gecko G201 and a 70V PSU and you will get near its maximum, but it will be far more torque at low speeds than you need or can use, and won't get to very high speeds at all.
Computers are quite good at maths. Don't worry about weird feed per step sizes. With those big slow Nema 42's you won't wont to gear down.
Forget feeding Z as a CNC through the Rack.
Backlash on a lathe is not as much of a problem as on a CNC mill, and you can live with it. It really matters if you turn curves which passes through a deflection point (turning a barrel for example). You will also find it hard to turn really precise bores, such as bearing seats if the backlash is not consistant.
The longer you play around with CNC machines, the more you see that the correct solution is anti backlash ground ballscrews. Yes you can make other solutions work, but this is the easiest, best solution. Ebay is the best source.
What software are you using? The spindle index setup will depend on this, as TurboCNC, Mach and EMC all use different encoder/indexer set ups.