Don't know about the 10.5 ring gear rearend, but the other GM's start with a .015 pinion shim and most of the pinion gears are marked + or - from there. Ex. if the pinion is marked -.002 you need a .017 shim and Visa versa for a + marking. In the end it all comes down to contact patern, but that's the starting point.
JMO
Zip.