[JWagner]

The convertible frame is usually much more robust than the tin top frame. I think that they use a different number of body mounts ,also.

 

[pictureguy]

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.

Michael

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[69SS]

Convertible frames are dimensionally the same as coupe frames but are fully boxed and have extra frame mounts. All 68-72 A-body convertible frames are interchangeable.