Easily the most useful tool in my assortment of bodywork tools is the shrinking disc. I’ve mentioned it a few times in J-L, including here six years ago:
Since that panel you’re working on is accessible from both sides it’s a natural for the application. The Wray Schelin design is very robust. Wray is to XK bodywork that Chuck Hadley is to E-types. Apropos, Chuck also uses the shrinking disc extensively. I suggest you look into it. It will make your job easier at the same time produce really a nice result.