Convertable frames are boxed in between the wheel wells to add support to the body and keep it from flexing.
68-72 used the same frame as well as Old's Cutlas, Buick Skylark, and Pontiac LeMans of the same years.
The station wagons and El Caminos have a boxed frame but are longer than the convertables.
Good luck with the frame. I am at the same point with my 71. Mine is shot and I need to find a new one.
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