The stringers won't be hard to locate they will be on the chines and centred between. The chine stringers are the ones most likely to cause grief as they are fully welded and sometimes the limber holes get welded shut,
l personally don't like the idea of insulation unless your in extreme climates. As a matter of fact as part of refitting my 84 built steely I'm making a point of removing any lining panels at the back of cupboards etc to allow easy inspection
this pic should give an idea of what's behind the insulation