ac install colonial

i am doing a ac job in a colonial style 2 story house. i want to get a supply duct from the attic to 1st floor living room ceiling through the 2nd floor bedroom . i am undecided whether to just come through 2nd floor corner and boxing it in. which this is a strait shot run, could i come down thru the 2nd floor closet and go into 1st floor bathroom/laundary room and take out fire stop and get into living room ceiling some how for the sake of a corner duct to be boxed in masterbed room. i hope this makes sense. is there a 90 boot to do this, to gofrom square register to a duct to go in ceiling joists.