I've never looked at the G96 option as anything other than a 3:55 gear change for a Posi? I assumed all Z/28's were posi and if you didn't order a gear change it came as a G80 3:73 posi as default? I'll have to check on that as the axle codes on the original diff were covered with ladder bars and couldn't be read.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Positraction rear axle. Required in 67-68 for
4.10, 4.56, and 4.88 ratios. Required in 69 for
3.73 <span style="font-weight: bold">(except Z28)</span> or 4.10 ratios.</div></div>