I run Edelbrock IAS Performer shocks...and love 'em. They handle the road, corners, and the load admirably.
If you're looking for helper springs, there are countless options available... air is one. However, if you're wanting to stay "mechanical" rather than air, you've got an option in addition to the add-a-leaf variety...
SuperSprings makes a spring that doesn't deteriorate the stock ride (improves sway control from what I've heard) and kicks in when the additional load is applied.