I added one to my 72K5. I got mine long time ago from an online sale. Can't recal what it was pulled from but at one point in 67-72 GM did install a Driverside batt tray.
That kit looks good ~ drivers side is just a mirror image of the pass side tray
Oh and I use
www.hellroaring.com Iso/Combiner to keep both batts charged and the second batt is 100% backup or I can combine the batt power when winching heavy loads. Not cheep but awesome product!