That actually should be pretty easy because some GM trucks of that vintage came with dual batteries. I did a dual battery conversion on my 86 Chevy Crewcab dually 4x4 that I used to own. I bought the tray and bracket that goes under the tray from the local chevy dealer. Then I had to drill some holes in the inner fender to bolt the tray and bracket too. The nice thing was that the inner fender already had little dimples where I had to drill the holes. No measuring, marking and center punching, just drill where the dimples were. They were kind of hard to see but I saw a couple of them when I was holding the bracket up and once I cleaned the crud off a little there they were. You might find something in the junkyard but I wouldn't count on it, most of the battery trays are pretty rusty. I forgot, the second battery goes in the same spot as the stock battery expect its on the drivers side. And I made my own battery cables using 2/0 welding cable hooking two blue top Optimas together, I was using it to jump start 24 volt heavy equipment at the time. I had it setup to where I could keep the truck running and charging one battery with 12 volts and very quickly hook both batteries in series to make 24 volts.