On my Group 4 I know extra metal was welded around the lights and also in the upper areas above the quarter windows. I have a piece of metal kind of trangle shape and curved that I understand can be welded into the area above the quarter glass. I will fish it out of my parts stuff and do a photo if anyone is interested in seeing. I run my Group 4 pretty hard on occasion, and no cracks yet, and no brace or other stiffing devices used. So, maybe one of the keys is to build those areas up when restoring the car. It also has a metal bottom belly welded under the car, so maybe that could add some stability, I am not sure.
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