Advertise it as a 66 GTO convertible frame! To start with !They are the same! Also would fit 64-66 special,skylark, converts! Plus gs’s 65-67 ,the front cross member shape is unique to the 66! The 64-65
have a welded on pull tab! 66 has more of a duck bill shape! 67 has different rear spring pockets and
the rear cross member has a little different shape! Some 65 Gs frames also have small diagonal brackets
on the cross member! This was discussed here on the board! Might bring couple grand or more! , if rust free ( no pits) and damage free, no alterations! Buick is the only one to use a boxed frame on all models! 65-67 GS’s except the 67 gs 340 and gs California’s! The other gm’s only used a boxed frames on the convertible models as did the Chevy elco’s ! Buick even use boxed frames on the sport wagons!