sh*t, it's a chevy! Who knows what has happenned in the past, what day of the week it was made, Etc...Etc...
So, what I have done in the past, is just fab up a Panhard bar, 1-1/4" Minimum, use Stainless Heim Joints, get the 4140 Chrome Moly Tube from , say Art Morrison, fab up a couple mounts and tweak the rear end where you want it. Like this:
This is what I commonly do to trailers, have done with early chev trucks, done it with my Tahoe and if my new wagon needs it, it'll happen there too. I'm showing you trailer pictures for easy clarification. The bars are smaller here, It's not hard to do, just make sure your welds are structurally sound. If you don't weld, you'll have to find a shop that you trust to fix it right, but this is a sure way to locate your rear end. The longer the bar the better for arc range, but the longer the bar, the more limber it becomes, so it needs thickness to retain rigidity. This is an airbag, trailing arm, panhard setup.