I was researching this same topic the other day and I found it dificult to find information on the original bars and how they mounted.
Here's what I gathered...
There are two types of GM sway bars. One mounted to the underside of the crossmember and the other to the front.
The holes are visible in the picture below. Two holes on each side above the word "Advanced" and "member" and one on each side below the same words.
http://farm9.staticflickr.com/8226/8533448304_0fdca3f3e3_o.jpgThe older type of sway bar attached to all three holes on the front side of the crossmember and had special brackets that replaced the bump stops on the top side of the leaf packs.
The newer type of sway bar attaches to the underside of the crossmember in the lower holes below the words and matched set of holes toward the back side of the crossmember and attaches to the axle with it's own set of u-bolts.
In the same pic, you can see the difference between the the Chev and GMC crossmembers and how those same holes are positioned on both. The GMC bar requires angled blocks for it to work. ...or you could slide the brackets inward so they line up with the flat part of the crossmember and drill new holes.
Here are some pics of the old style Chevy sway bar I picked up last week for reference.
Entire bar w/ brackets -
http://farm9.staticflickr.com/8375/8496049577_730bde2be9_c.jpgCrossmember bracket -
http://farm9.staticflickr.com/8367/8496050031_1361313b71_c.jpgBump stop and end bracket -
http://farm9.staticflickr.com/8101/8497154920_8edb031007_c.jpg