Take a look at
Alcanspring in Grand Junction, Co. They have some informative tech on their site. Their prices are not much more than new OEM springs.
All the off-the-shelf lift springs fall short in some way:
1. Leaves too thick and too few in number for a rough ride.
2. No diamond cuts, tapered ends, poly pads, etc.
3. Compromised shackle angle because they NEVER make their springs for longer shackles as they should to keep the shackle at the stock angle and still allow for longer lift springs' full spring compression without causing the shackle spring eye to strike the frame or bracket.
I could go on, but I'll just mention that I think the best 67-72 Chevy 4x4 spring lifts involves custom springs made with
Offroad Design extended shackles in mind to keep castor within specs while allowing for smooth suspension cycling.