Jeriah,
You have probably noticed that most of the Chevelle frames have the c-channel portion below the doors caved in (bent upward) from people putting floor jacks under them. Boxing that section prevents that. Also, the frames twist quiet a bit under high torque loads. That is the reason GM originally used a boxed frame in the '65 hp big block cars (Z11), but later decided it wasn't required. The boxed sections in the El Caminos and convertibles are plenty beefy and they add a "factory" look, but the frame could just as easily be boxed with steel plate.
Pappy
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