The 66 Skylark convertible, and Skylark GS frames both have the boxed side rails like a '65 Skylark GS.
Sedan, hardtop, or convertible doesn't matter as long as it's a GS frame, or a Skylark convertible frame.
It's nearly identical to the 65 frame EXCEPT the front crossmember looks slightly different......the 65 has a welded on 'point', while the 66 'point' is molded in. It's not a functional difference, just cosmetic.
I'd suggest just having your frame fixed if it's only the front horn that is bent a little.