I thought the box was not unique, and that the the pitman arm was the unique part for your Malibu.
The GM PN for the box is: 7841893 (1978-1980 manual gear)
The GM PN for the pitman arm is: 7837644 (same as V6 with 605 PS box, up to about 1981 or so, then they used a different pitman arm with the 605 box...go figure)
Here is a site that cross-references the steering box to other part numbers; perhaps you can find one with one of those cross-references.
GM 7841893 Steering Gear, You Can Buy High Quality GM 7841893 Steering Gear from yoyoparts.com
If you want me to look in my Hollander book, I can do that later.