The Hemi convertibles also had two rocker panels. One that was thicker (if I remember correctly it was 16 gauge metal) and inside the one we see. That, in turn, was tied to the torque boxes.
In a unibody car every piece of sheetmetal in integral to the strength of the car, but there is a frame, per se, which ties everything together and lends strength. Those are: the frame rails, crossmembers, and rocker panels. By tying in all four rails with the connectors we can stiffen things up and torque boxes allow for a better tie-in with the rockers. But the rockers are still a weak point. Hence the reason Ma Mopar double up on them in Hemi convertibles. Every other part of the sheetmetal structure welded together, and usually overlapped in the process, keeps adding the strength, like the inner aprons and core support tied into the front rails.
While our FMJ's (and Y's) don't have the torsion bar crossmember, we still have a crossmember tying both rockers with the floor pans.
If you're worried about body flex beyond the added frame connectors and torque boxes you can do what Ma Mopar did with the rockers. You also have the option of going in on the frame connectors and putting in an x-brace. This is what the Brand-X full frame car convertible cars came with.
XV Motorsports also proved out on the chassis dyno that putting in a reinforced brace on the core support does a lot in tying together the ends of the front rails. You can google XV and see the brace I'm talking about.
You can also go in and run an X-brace between the rear rails that shouldn't interfere with the tank.
Personally, I don't see the need in the X-bracing, just an idea to run by you if you like to over-engineer things.
My M-body will be seeing the rocker "upgrades" since I want to stiffen it up that little extra. Also, it will see the usual frame connectors and torque boxes, as well as the added bracing to the core support. I want the car to be able to plant the power to the ground on curves.
