Re: Convertible sag ???
First, do you know the condition of your rockers, and whether the extra vert braces (one bolt-in "dogbone" under each rear interior panel and the bolt on X-brace underneath) are in place?
I had the same problem on because of rotten rockers on the 68, and actually had the rockers starting to stress crack at the front where they join the cowl. This was 5 plus years ago, before the convertible rockers came out in repro. The only thing I could do was weld in frame connectors. This helped with the overall structural rigidity, and took care of the sag. The 69 has good rockers, so I haven't done anything with that one.
If you're not planning significant horsepower and want things to be "original," the new rockers are the way to go.
69 'vert project big block/TKO 600 RR
68 'vert driver RS clone -- gone!