[Jeff Lee]

While I can't offer any advice from experience, I would think that the OEM added structure to the frame to replace what is missing; the roof structure. Since your adding a CM cage (good move on the CM) and other common upgrades, you are now making it better than if it was a hardtop. I can't imagine you need anything more than the guy with a hardtop.

By the way, I once had a '67 Camaro convertible. I bought it in the '80's to flip it for a profit. I was excited because it was red on red. It was a 327 2bbl PG so I didn't care about numbers, it was just a base Camaro. With the top up, it rode "OK". With the top down, it had so much cowl shake over RR tracks it wasn't even funny. And it rode horrible. Well then I discovered that Mustang &Camaro convertible CONVERSIONS were once popular and this was one of them. So it didn't get the upgrades or the shaker cans in the trunk. That shows you how the OEM upgrades are needed for a true convertible.
And I got real lucky on that car, broke even with a guy that didn't care about the "upgrade".