I suggest an index wheel and pointer on the back shaft of the steppers. If you simply write down the index position for each axis when you zero out the axis you can easily get back there after collision, power off, or the next day.
Not to discourage you, I calculate 118 lbs of HOLDING force, which is at zero speed. Look at the torque curve for you motors at 400 rpm (200 ipm) and at the supply voltage you have, and I think it will be 25 to 30% of that. Skimming a plasma torch over the surface without touching is a lot different than pushing a cutter through aluminum.