Chassis slap is one of those things you kinda have to tune your suspension for. A little slap isnt bad but to much slap and you start busting things like fan blades and motor mount screws. The flip side is if your suspension is stiff enough to have no slap on really big jumps and the rig will...
You may want to eventually look in to one of the "indestructible" bodies on ebay and sounds like you may need some thicker shock fluid and/or stiffer springs.