How to reach such extremely long suspension travel?

Spend about $10k on gear from Superior Engineering.

 

If you look closely youll notice its not all that much flex in the grand scheme of things, notice spring over axle (+6inch lift) and modified shackle mounts so theyre futher from the chassis (+2inch or so) in short, soa some mount mods and long shocks from whatever you can get length in you have this.. longer springs may also get you a bit more out of it but thats mainly whats going on here, big lift and an unstable car

 

A custom set of leaves re-curved with a nice progressive tapered leaf set will do it easy, keeping enough set in the rear part of the leaves to reduce torque wind up if you have serious power.
Or you can remove some leaves and fit coils between diff and chassis for the best of both worlds. Dont use long shackles it just stuffs the whole geometry up and raises the rig for no real gain if you are not changing the main leaf for a longer item. With a bit of know how and experience with attention to detail you can easily get that sort of travel and still keep nice on road manners with a respectable ride.

Many underestimate leaf springs thinking full link systems with coils or coil overs are the way to go. A good leaf setup is always a cheap system compared to the components required for a good coil link system.