The entire lens for '69 quarter light fits through the quarter panel, there isn't a hole just for the light socket. The outer edge of the lens can not be used as the template for if you do, the area cut out will be too large and the lens would merely fall through. Try the inner edge of the gasket on a piece of scrap metal before doing any cutting on the quarter. You also need to put the light in the proper position else if it isn't it will be a sure tell-tale sign of a clone.
1969 GTO Restoration