Movable partition systems include a movable partition, a soffit positioned along an edge of a track, and a molding member engaged with the soffit opposite the track. The molding member is at least partially abutted against a wall and configured to be adjusted laterally relative to the soffit. Methods of installing a movable partition system include suspending a track from a support member, inserting an end of a soffit between first and second ends of a molding member, and coupling the soffit to an edge of the track or a wall of a header structure. Methods of forming a header structure for a movable partition include disposing a track for a movable partition at least partially in a header recess of a header structure, engaging a molding member with a soffit, and coupling the soffit with the track and between the track and a wall of the header structure.