You defo cannot put Mark 4 headlights onto a Mark 3. I know because I tried it years ago. I cut out the headlamp surrounds, managed to secure them in place, fitted the grill around them. Stood back and it looked good. Then when I put the bonnet down they are the wrong shape and the bonnet will not shut by a good half inch. They will not fit and Mark 4 style are the only way.
