My chal convert is a fairly low mileage car and still lacked a little in the structural integrity department. I added heavy subframe connectors as a precaution since the hemi weighs almost 800 lbs. I think that it is safe to assume that you will have structural problems unless you have someone build a total subframe style undercarriage. You could start with one of the new front aftermarket subframes and go from there.
I cut up a 71 chal convert shell a few years ago (wrecked and rusted badly) and I was amazed at how thick some of the components were on that car. I thought I would never get that rocker cut all of the way through. My gut feeling on this project is that if the roof is bad, find a replacement and change it.
I have a 66 coronet 500 convertible that runs and drives that I will take $5000 for. It has power steering, power brakes, power windows, a/c, buckets, console, factory console tach. The right quarter has some bondo, the right lower fender area is rusted (repairable) and there's rust in the back floor board area, but overall, would make a nice resto. It's not a rust bucket. The frame and cowls are solid. The trunk only has one small area of rust...easy repair. The only rust repair panels that it would need is the back floor pans, maybe. Was a 361 but now has a 400 auto. You could drive this car while you restore it. There's a new convertible top in the trunk. Oh yeah, and the power top still works great. Like I said an easy resto. I just don't have time to restore them all. Clear Texas title. If anyone's interested, I'll post some pics.