Stu, the screws under the side skirts are machined and screw into rivnuts mate, best thing would be to soak em in wd40 (or summat better) then try n undo em, BUT chances are they are rusted in, so as Lee suggested you could try drilling them, though do it VERY VERY slowly, as soon as you put a bit oof speed on it the rivnut will turn with the screw and drill, in the sill mate, which will in turn just melt through the skirts... I was lucky with Tony's some came out no prob, some a drilled a couple I was able to grind the top off the screw then bang the rivnut in and replace after painting...

Best to use the proper Ford Rivets, as these are plastic coated and will prevent corrosion when repainted/reriveted in place...