As far as the 60" Suburban springs go, Tom here at ORD runs them and his truck has PLENTY of flex. 4" Tuff Country EZ rides in the front, 60" Suburban springs in the rear with a shackle flip. He was on the Ultimate Adventure a couple years ago and also on Xtreme 4x4 etc., it's the white pickup with the tube bed.
As mentioned before, the longer the spring the flexier it will be (more leverage). This also means more axle wrap (again, leverage) and worse clearance. There are always x-factors such as the way the spring pack is tied together, number/thickness of leaves, etc.