Rob, u can add crossmembers all day long, but u MUST add metal to the frame to carry the down load created by the weight of the engine. The span is the distance between spring mounts. The stock frame WILL NOT carry the weight of the engine in the center of that span. "twist" failure due to the engine torque is completely different than "overload" failure. Crossmembers/stiffeners, etc only address twisting.
Heavy springs will make the effect of the overload on the frame even worse.
Plastic trucks suck...
Iron trucks rule...