Inner rocker panels
Ray is absolutely correct about the inner rocker panels. You had better replace them as well while you're putting in the new floors. They alone are the longitudinal structural members in a convertible.
I helped a buddy restore/rebuild a 73 Mustang convt. What a chore that was! The only exterior panels reused from the original car were the outer rocker panels and the rear deck panel (between the top and the deck lid. This particular car needed front and rear inner fender aprons, radiator support, both quarter panels, inner and outer rear wheel houses and the tail light panel in addition to full floors and the inner rocker panels. Bolt-ons like the doors and front fenders were also replaced.
Was all this effort worth it? I don't know. I've junked better conv'ts than the one he fixed. The restoration took us the better part of two years. But he now has a 'twin' of his '73 Mach 1, but in drop-top form. Both are 351C-4V 4-speeds. When I said twins, I wasn't kidding -- right down to the paint color, wheels and stripe (although the convt stripe doesn't say 'Mach 1').
I have driven this car on occasion, and believe me, you don't want to put your finger in the door/quarter gap while driving down the road. The body has enough flex it is scary. If we were to do this restoration again we'd probably use cr/mo rectangular tubing in place of the inner rockers and also tie the front and back halves of the car with subframe connectors.
I don't mean to discourage you, just giving a note of precaution.