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The best flex will be achieved by custom designing a link system with coil-over shocks. But like with any design, there's drawbacks. You have to cut into the tub for the upper coil-over mounts. With all the adjustability comes constant tweaking to get it right.
The best bang for your buck is SOA, Revolvers and a ladder bar. I'm not sure I'm on board with the z-box, but if you don't mind experimenting, go for that too. This would be mostly bolt on, with the exception of spring perches, ladder bar mounts and z-boxes.
Your "sand" criteria scares me a little. Do you do a lot of sand, or is it just that if a sand hill comes along your path you want to goof around on it? See, spring wrap is the major drawback of SOA. Sand hills and SOA don't get along very well, even with a ladder bar. To build SOA strong enough that you can blast up dunes, you need a ladder bar next to each leaf spring and that will limit suspension flex. You kind of have to build your rig towards the wheel'n you like to do.[addsig]