With no shims you have zero caster...and you really don't want that. The factory (non-power steering obviously) spec can be found in the manual but ought to be a max of 2 degrees (+/- a small fraction). If you don't have enough caster, your truck will steer much more easily but will wander all over the place and it will not straighten after a turn very well at all. Some people who think taller back tires are the answer and who never change their caster to adjust for this create trucks which are unsafe. Raising the rear lowers the caster angle every doggone time. The same thing will happen if you use smaller tires on the front. If you increase the caster to 4 degrees with manual steering, your truck will drive straighter but will be harder to turn...especially in town. The reason why is caster tilts your axle back (or front if your vehicle was built with negative caster) and so to turn it, you literally have to lift your truck up. If you want a nice visual example of caster, just look at a motorcycle from the side. 6 degrees might work just fine with power steering, but very few people anywhere will be strong enough to steer a vehicle as front heavy as a truck like this with that much caster with manual steering. And the greater the caster degree, the more wear on your kingpins, kingpin bushings, bearings, etc. That part is a given. There is a good balance which must be established. Worn out springs (leaf springs of course in this case) can also cause caster angle changes if the arch has been bastardized with time. And it can change with age, rust, use, etc. So just sticking some shims in there is only a start. Get the truck to a front end man who has some gray hair and knows what he's doing. Remember the engineers at GM didn't make it this way because they were stupid. They did this because they saw it as the best balance...and they probably never thought anyone would be driving these trucks at freeway speeds.
Also. VERY IMPORTANT. You have to be totally cognizant that there is a pin on your spring which mates with a hole in your axle. That pin is the end of a bolt which holds your spring leaves together. It is imperative that pin mates very well and very securely with that hole. If you don't pay attention to this and if your shims are not correct, they can raise the pin to the point where it just barely or worse doesn't connect with that hole. The best of all worlds is the new shims you use have the same diameter hole as your spring pin's bolt, are held in place (screwed down to the springs) with new spring pin bolts and therefore the bottom of the shim has 100% of that pin available to fit into the hole in the axle. NOT ALL SHIMS ARE MADE LIKE THIS. Some are made with a hole large enough for your pin to slip through (meaning you don't have to change pins when you change shims), and this obviously lifts the pin off the axle more as you increase shim thickness and at some point you can have wall to wall trouble. If this is in any way confusing to you, STOP and study it and ask for help.
