Aluminum is too temperature sensitive for a back plate.
Machine the back plate to fit your spindle but leave the OD and face .025 large and then mount it to the spindle.
Finish machine the face of the back plate on YOUR lathe to fit YOUR chuck and it should be dead nut.
If there's any runout, it's in the chuck then.
I agree about the six jaw. If you can afford it, that's the way to go. You can hold very soft parts with very little tension with a six jaw.
Flying chuck keys will make you st,st,st,st,studder!!