Well, I think that you need more than you might think. I'd recommending an experiment. Find an immobile ground object and chain the FEL to it. Now, you need at least sufficient Rear weight to pop the hydraulic relief valve BEFORE the rear tires leave the ground or get real light. This is your minimum...for me, it's about 1000 lbs plus both rear tires loaded...another 1000 lbs.
Now, the rest is up to YOU to drive safely. GO SLOW when turning corners particularly when FEL is raised because this DRAMATICALLY raises your center of gravity. Add centrifugal force and you will never be able to make a safe fast turn with heavy FEL load swinging on a chain, regardless of rear wheel width or 3PH ballast.