The frame under a 1969 Chevelle convertible is
unique to the convertible body style. It's *not* interchangeable with a coupe or sedan frame. While the general shape is similar, the convertible frame has reinforcements and modifications to account for the lack of a roof structure and the added stresses of the convertible top mechanism.
Therefore, you'll need a 1969 Chevelle convertible frame to properly fit under a 1969 Chevelle convertible. Using any other frame will require extensive, highly specialized, and likely impractical modifications.
