is there any documentation anywhere of the way the lower rocker panel seam on my 1969 firebird convertible is constructed. Mine is looking a little worst for wear now, it is swelling open. It looks as though there should be several layers of metal all joined together at that seam.
Search the internet for Rocker panel replacement. The rocker on a convertible is usually of heavier gage than coupes.Its not an easy job. there are a couple of posts on the Corvair forum on replacing rocker panels. The rocker on convertibles is critical for structural integrity . In replacing the rocker panel you will have to strip the car interior to some extent and weld in bracing to support the shell while you do the repair.The body becomes quite flexible with the rocker panel removed. You will need cross bracing and fore and aft bracing.Take measurements of the opening so you can check your work. I would address this issue as soon as possible. If its a small problem now you want to repair it before it becomes a large issue. Hopefully it's localized. Rocker panel replacement assumes a worst case situation Rich
I second that motion: be wary of the flexible convertible body and add bracing.
I just had my convertible floor boards replaced and quite a bit of other flooring cut out and replaced. $3000 total.
Upon getting my car back my doors no longer lined up. I had already replaced the driver's side hinges and that was perfect. I had to readjust the hinges and then repair the passenger side. I was able to get everything pretty close but not perfect. If I moved it one way the door would hit the rocker. If I moved it the other way the door would bind with the frond fender. Nightmare.
All this was a result of just 3/8" of sag.
So my suggestion is to check the alignment of your doors periodically as you cut metal and weld things back in. You might be able to locate a jack under the rocker or frame and lift while welding in order to get the alignment perfect. Other bodymen might know better than me.
Another suggestion is to raise the car and support it on the frame rails. spread the load over the rails to keep sag out of the picture. You can use jacks to adjust any sag out as DanM suggested. take floor to body measurements . Record them for reference. The doors should be removed as you will weld bracing there. Again this assumes a full rocker panel replacement.
I did a search on youtube look up 66 Chevelle rocker repair. The website for the poster is Allisoncustomsonline.com He has the car stripped which is the bestway for a major repair of this nature. Hopefully you don't have to go that far. Although I would take the front end and engine off the car. The less weightyou have to cause bending or sagging the better. Is the rust localized?If it is all it shouldn't require all this extra work. Best to check both sides. Maybe take an awl and sound areas that are not visible to the eye. Rust is quite insidious.