Charlie, what's the weight of your gantry complete with the spindle? If you put that weight into the motor calculator spreadsheet you'll be able to see if one Nema 23 will do it. On my gantry it was going to be only just ok (I've got more plate in mine than you have as I believe from the drawings you're using alu box sectio to mount your rails to?) so I went larger to be on the safe side. If you're borderline all it will mean is that you won't get the higher speeds of velocity and acceleration.