You could do it it vertical mode, I suppose, but you'll need to make a fixture that holds onto that aluminum bar under the centerline of the rotary table faceplate while spacing it far enough away that the head of the mill can reach it without banging into anything.
You don't say how much of that arc you need, but if the stock has to remain 6" long it seems like you're not going to get to too much of it before the end of the aluminum bar hits the edge of your mill table.
A better way would be to use the rotary table in horizontal mode, held down to the table and facing up. You'll still need to provide some fixturing, but you'd minimize the clearance issues. Cut an arc-shaped slot inboard of your final dimension, and then creep up on it slowly...