Probably 3 or 4 good supports, 1" X 1" running crossways just below the roof, and another one front to rear. Then 2 or 3 braces on each side running at an angle to the opposite side on the floor. Big thing is to get the sides, door jambs, front and rear from moving when the old roof is removed.... Shut and latch all the doors, might even want to use some straps to temporarily tack weld them shut. Keeping everything and anything from moving is the key to keep things in position...no such thing as overbracing....