Rust around a vinyl top is no joke. I plan to do the same on my 72 Supreme. When the glass man came to replace my windshield, he told me (over the phone as I was at work) that the channel was so badly rotted that he could not warranty it against leaks. The top was replaced once and the pillars were put on AFTER the top giving water a channel into the metal. And yes, in a good rainstorm, it rains inside too. Good thing the floor is rotted a bit to let the air circulate and dry the carpet
The rear window where it meets the trunk is about gone right at the corners. They dont make replacement panels for that area. It is a custom job that I have to find a good lead man (as in the chemical element Pb) to fill it, OR learn the art myself.
I'll just say that I am SCARED as hell to remove that top until I got 3 times as much cash as the body man tells me it will cost. And I am thinking it will cost....
Just my .02