I have a 1988 ford f350 reg cab long box I use as my primary wheeling rig. It has no lift, no lockers, 33X14X16.5 boggers, 460, 4.10 gears, and a zf 5 speed so I am pretty limited as to how far i can push it. I have noticed a large amount of frame flex and before i modify axles/suspention do i need to worry about the frame cracking or bending, and if so whats the best way to reinfoce it. Anyone who has gotten hard core with one of these truck and has advice i would love to hear it.  

Just about every long bed p/up flexes like this. The 3/4 and 1 tonnes tend to show it even more than the 1/2 tonnes due to the stiffer sping rates.

You can let it twist, or fully box your frame and add some crossmembers.

That isn't much twist for a long bed in the. I can make my bed almost touch the rear wind in my k1500. The old k20 chevs used to show some sick frame flex, box would chaffe the paint off the cab.

Its no big deal.

most longbeds do it... if you want to reduce it, i would agree.. box the frame and add more crossmembers.  

that pic isnt really showing how much it can flex, the only times i really worry is when my box touches the rear window gasket.  

Softer suspension springs would help, as I bet you get frame flex before tires are stuffed in the fender.