Cool car. I had a 66 convertible up till last Christmas. I went in the shop and could hear it arguing with my Camaro. Sounded like a married couple. Seriously though, mine looked rust free, but had a ever so slight door sag. Ended up being the inner rocker support was rusted out all the way down. Looked great from the outside though. Doing what you are planning definitely support the car. The it together, totally different beast than a Camaro. I would cut a small inspection hole in the front of the rear wheel housing to look into the rocker. It is a easy cut/ fix. Cool car.
1969 x-77 Z/28 orange/ white stripes, named by my daughter "Clementine ", M-22 close ratio, 4:10 12 bolt, CE 302. Hugger orange with white stripes. White standard interior. Restoration complete May 2018