600oz/in is a fairly powerful level!
Are you sure you actually need that much or are you just worried about erring on the side of caution?
As a comparison, the 60cm x 90xm CNC machines from China commonly use a mix of NEMA 23's and 34's, at 260-around 380oz/in or so. And these are more than adequate for almost everything from wood and plastics to aluminium.
420oz/in or thereabouts seems to be the common sweet spot for the G540's given the 3.5A current limit, and to work within the parameters of the stepper itself for maximum speed and torque performance.
This vendor has a model you could run in series mode to be right on the G540 limit:
Nema 34 600 oz/in Stepper Motor
This guy in OZ has the same ones:
But i'm sure you'll find plenty of others around. I just wanted to mainly raise the point in case you haven't actually sized the correct rating for your requirement, that in going up in size dramatically, it can actually be a trade-off in the torque/speed/performance/power used balancing act, and could actually have an adverse affect. Bigger is not always better.