Will seats from a 2003 beetle fit in a 1974 super beetle?

As a straight swap, No. Every few years VW changed the seat mounts let alone different models. But as far as retro fitting them, people do it all the time. People have put seats from Hondas, Toyotas, etc. in classic bugs. It's just a matter of making your own mounts. Just measure the width and height to make sure they will fit.