The ES71, 70, 51, 50, and 30 are all v2 Octalink spline patterns and all will work with your FC-M751 XT cranks. A 113mm spindle length will get you a 47.5mm chainline, and a 118mm spindle length will get you a 50mm chainline. I don't know enough about your frame to know whether it has a preference for the 50mm chainline, but this is often a safe bet.
You can use the 73mm assembly on your 68mm shell by installing 3mm worth of spacers underneath the drive side flange. If you install the 73mm cartridge without the spacers, if it is the 113mm spindle length, you'll end up shifting your chainline in far enough that you may have difficulty shifting to the granny ring. If it's the 118mm spindle, your chainline will be where it would have been if you had a 68x113 BB installed. In either case, you'll offset your whole crank assembly to the left by 5 millimeters (probably imperceptible to the rider).
You can use the 73mm assembly on your 68mm shell by installing 3mm worth of spacers underneath the drive side flange. If you install the 73mm cartridge without the spacers, if it is the 113mm spindle length, you'll end up shifting your chainline in far enough that you may have difficulty shifting to the granny ring. If it's the 118mm spindle, your chainline will be where it would have been if you had a 68x113 BB installed. In either case, you'll offset your whole crank assembly to the left by 5 millimeters (probably imperceptible to the rider).
