The lower radiator braces don't really keep the frame rails from widening or getting narrower, that job is taken care of the K-member. The lower support counters the frame rails from twisting and moving up & down in relation to each other, when engine torque and/or cornering forces caused from the wheel under load puts a lot of force to the suspension and frame rails.
Those forces cannot be controlled with a simple bolt-in bar mounted in between the frame rails.
