Something like this :
In addition to what @brightspark said regarding the accuracy of the boss, there is also the likelihood that the jaws would then be too wide apart and the flange would be resting on the next step down and thus making the face below the jaw line.I agree with holding as shown above. But can you not just hold it on the boss shown, and push it back agaisnt the jaws, skipping the home made arbour.
depends on how accurate u want it to run. if its held in with the shaft it will run true after machining the face
will be fine taking a light skim off itInitially I put the boss in the chuck and set it in with the disk against the jaws and the disk was all over the place,which is why I decided to turn the bar down and mount the assembly the same as it will be on the tractors driveshaft.
Maybe the disk is warped/damaged.