Best cheap springs you can get are Moog 5716 for the C2500 with the big block or diesel for $100 new. They're about 1500 lb/inch, and then cut a coil off to get the ride height back down (which also further bumps the rate up some). It sounds stiff, I know, but it works unbelievably well on my Tahoe and at least at the front end, the Tahoe and the C1500 are the same. I think stock C1500 is between 850 and 1000 in the front, stock, depending on the year and engine. Most of the drop springs that I've seen are barely stiffer. The trick is finding shocks that can control them - the best I've found off the shelf so far are the newer Chevy Express 3500 stock front shocks since they have a similar stock spring rate and motion ratio, though eventually I may build a set of QA1s.
I second
@sewlow's G80 comment. The G80 locks pretty abruptly, which you can deal with, but it unlocks above ~20 mph. I know there are sites that say it acts like a limited slip above that, but that is patently false. It's an open diff once the speeds go up. I've also discovered that the G80 does odd stuff under braking. The Tahoe has a big rear bar and overly stiff leaf springs, and you can lock up the inside rear if you're trail braking. In slower corners, if the contour of the road is just wrong....this can actually make the G80 lock up on deceleration. Chirp, BANG, instant oversteer. It's really exciting, in a bad way, and really unpredictable.
I think I'd stick with the open diff over the G80; I've only kept the G80 in mine because I haul dirt bikes with it and 100% lock can be critical getting out of muddy parking lots. In a heavy Tahoe, it's easy to drive around the odd behavior. It's much harder when the rear is so light in a truck.
I'm not sure if the last of the B body chevys have the same spline count, but if they do, those all had clutch-type Auburn limited slips that would be much better for what you're doing.
EDIT: also curious what you eventually come up with for arms. With halfway decent tires, I'm having a hard time keeping the eccentric bolts from moving around on me.
