Th "XH" box is from '86-'92 Camaro IROC & Fireirds with the WS6 package. '80+ GM cars share pump lines. For '79 you'll need to either get adapter fittings or swap pumps & lines from an '80+. The box itself is a direct bolt in IF you already have a "4 bolt" 800 box. Just reuse the existing pitman arm. If you have the "round top" 605 box, you'll need to get a G-body 800 Pitman arm, too.
Tommy