I have coils.
I am considering lowering the rear a couple of inches to level the truck out or 2" in the front with coils and 4" in the back.
GMC Pauls sells lowering springs for C20's.
http://www.gmcpauls.com/lowering_springs_&_parts.htm
63-72 3/4 Ton Front Lowering Coil Springs Pair
4145722 For 67-72 3/4 ton trucks, Heavy Duty High Performance Front Lowering Springs Available in 1", 2" drop. Sold as a calibrated matched set. These are not cheap imported car springs like others sell these are custom made in the USA and are calibrated matched sets made specifically for the truck they will be installed on. $129.95
63-72 3/4 Ton Rear Lowering Coil Springs Pair
4145732 63-72 3/4 Ton truck Heavy Duty High Performance rear lowering springs. Available in 2", 3", 4", 5" or 6" drop. Sold as a calibrated matched set. These are not cheap imported car springs like others sell these are custom made in the USA and are calibrated matched sets made specifically for the truck they will be installed on.