I am converting a tub to shower. I have moved the drain and converted to 2". Now my problem is I have Steel Studs and concrete floors. Do I place Hardi board to floor and then do the pan or do I place 2x6 in between the metal studs do pan then hardi. The reason I ask is that it would be impossible to get the boards between the studs to be plum and where the steel studs attach to the floor stud there are screws that hold the studs.