What is the best seam sealer for between the quarter panel and the panel below the window on a 1970 Cutlass Supreme?

 

If you mean the access panel attached with four screws, I think olds factory used butyl tape. It will seal it up quite well, but still allow panel removal if needed. Sometimes takes a little pry bar to do it


If you mean something else, 3M makes several varieties, most catalyzed, that would be considered a permanent joint. Set up time is about a half hour, or thereabouts.