Yes I think you are right there is little difference between the interiors of a 240D and a 300D - coupe models are sometimes slightly different though. But I can't see why they would have made too many complications during manufacturing. It is easier to fit one part in many different models (if you see what I mean).

If you're not sure take some measurements and maybe even a photograph if necessary.

Look out for height adjusting seats - I've got one fitted as standard on the driver's side of my 300D but there isn't one for the passenger.