Now a thread that I can actually help with. Are you running leaf springs all the way around. Spring over or under? Do you want to stretch some for a longer wheel base. If you are wanting it for a rock crawler, you want to keep a low center of gravity. For best flex, go spring over.
When I had a jeep yj, the best junk yard combination for flex and durability. I took factory springs mixed with top leaf from an XJ or s10. XJ with stretch rear another 4 inches and flex is awesome. Rancho 4044s on the front will stretch it up front 2" with good flex.