I don't think a convertible frame is more "substantial" than a hardtop - it's just boxed.

The hardtop frame is a C-channel and the convertible frame is boxed (like the C-channel is closed-in).

Don't think I'd try to modify a hardtop frame for a convertible unless absolutely necessary - especially when you can pick up a replacement frame. I've seen some convertible projects (on e-bay ) in the $1-2 K range - if it's got a good frame - you could probably part it out and darn near break even.

my $.02