Hi Chris,
Sid knows exactly what he is doing, so no worries there. If front is lower than back, your caster angle will be reduced, so if he has you using 2.5 degree shims, you're very close to stock or certainly in the range. As I've said many times I prefer a little more caster to force the wheels to return to straight out of turns and to make them stay between the fence posts while I'm driving. If your back is lower than your front, your caster angle will be increased (obviously, but I find a whole lot of people will put larger tires on the rear and never think about what it does to the steering). If you put something very heavy in the very back part of your bed and it squashes the springs down, you may find your truck steers really easily (from the weight shift) and stays straight (from the caster increase) but of course this isn't a safe steering situation.
In the 80s a friend put some huge tires on the rear of his truck and some on the front that were maybe 24~25 inches in diameter. He told me one day "I like the look and the thing has never steered easier but it wanders so much I'm scared to death to drive it." He had created a generous degree of negative caster with the changes. Something else not everyone understands is positive caster will additionally reduce the amount of negative camber increase while you're turning. Which is really what you want happening up there.
The old truck axle if you look at it has a ridge on top where it was cast and so does the bottom side. And then where the springs mount it is nice and flat. So there is no flat side on which to get a level measurement anywhere but where the springs mount. In addition to this, the top strength "lip" (angled out ridge) and the bottom one are not equal, so using a level or a t-square will give you wacky results. So yes, definitely to answer your question, the king pin angle (which determines your caster) is all we need to care about. The stock axle wasn't really parallel to the ground (if you look front to rear). If you had somebody to mill the bottom of it so that it was perfectly perpendicular to the spring mount area, then it still wouldn't be parallel, either. Because once we put a shim in there it would tilt backwards by the thickest part of the shim, giving us the positive caster we need.
Hebert...Can you soften the springs and keep the stock height? Yes, I think so but you need to talk to a really good spring guy. You're going to need to have some springs made for the sbc engine's weight which preserve the stock height...and maybe they're already out there. You could use Google and search. Or possibly some spring guy would know how to determine if you could safely modify the stock springs. I don't know diddly about making springs, but I agree with you on the stock height. That is one of the things I like about the look of these trucks. I'll bet Jerry knows about springs. Ask him and see...
Jon
