Cant help ya on the spring rates. I'm guessing you'll have to mock it up, and weigh the front half to give the dealer some info. But, my question is, why wont air work? Hell, every semi is running on air, and a hell of a lot cheaper to adjust.
May not be a bad thing at all. The weather has been cool and moist enough in the mornings that water is perfectly normal, and the black may just be old ash laying in the pipe, or you're a tad rich due to the choke and colder weather. If it's still running good, I wouldn't sweat it much.
Hard to tell from your post, but the dreaded death wobble only starts at about 45mph. Did you lift it? Sounds like the caster angle is off, and an alignment will cure the problem. The more ya post, the more you'll learn round these parts. :D