the 550 is open enough that you could set the boolit in place with your right hand [by reaching through the open frame] and with enough flair it will stay there through a smooth press stroke.
you can also place the new case in the holder and turn the shell plate with your right hand.
your production wouldn't be 400 per hour, but neither is mine, and I have a lot of practice with this type of set-up.
I'd think even with one hand you could turn out close to 100 [completed rounds] per hour with a good set up of the [don't have to have Dillon] dies and a little practice.