I'm not familiar with the C20 suburban suspension. I'm assuming they're the same as the pickups?? I'm also assuming you have rear leafs?? But check out
https://westernchassis.com/home.php
They have drop spindles, axle flip kits, 8 lug disk brake conversion, etc...and all for C20. Browse their site throughout or call. As far as I know, they're the only place that has a focus on C20 as well as C10.