Confused yet?
Let's get the terms straight first, for everyone.
http://www.ozebiz.com.au/racetech/theory/align.html
Ok, from the factory, these trucks only allowed for toe adjustment thats it.
Now there are a couple of things you can do to adjust castor and camber.
Castor:
If you buy, say, the extra 1/2" long rear shackles for the front springs, your changing the castor slightly. Just slightly though.
beater_k20 mentions a shim, but that is highly not recommended, and downright dangerous. I've seen some welded shims, those are ok, but that places some goofy loads on the axle that it was not designed for.
Castor & Camber:
It's rumored, although I've never seen them, it was written in the Moog instructions on a ball joint I bought once that it was available, but I looked and coulnt' find them. There is supposedly an eccentric sleeve you can buy for the upper ball joint. That sleeve screws in (with the special tool) and new ball joints usally come with new one (a regular one). An eccentric sleeve could change both castor and camber in theory. Here's why I don't think these really exist. It seems it would be a major pain in the a$$, especially for a DIYer, to get it right. It would require that you install the sleeve, assemble the rest of the knuckle, put it back on the rack and see where it is. It would be a total trial and error thing with dissambly and assembly of the entire spindle and knuckle required to get it to the specification.
Camber:
You can install a shim between the knuckle housing and the spindle. It's a graduated device so take it to a shop before you dissamble and see where your numbers are at, install the shim per the instructions to correct the camber from the numbers you got from the alignment place and re-assemble. Take it back to an align shop if you want to check, but it should be right. Readjust toe and your all set. I've done a couple of these and these work well for camber adjustment.
This is the one I used:
http://www.ingallseng.com/parts/26126.htm
Here's the speicifications for our trucks straight from the GM ervice manual:
Look at the KA10-20-30 specs. C is 2wd, G is van, P is forward control chassis