I have been told that these are not directly interchangeable however I have seen a 260Z with 240Z tail lights, what exactly is involved in fitting them?

Thanks

Well, first you have to find a set

Other than that I think the actual metal they screw into is different (larger hole in the 260Z) so it might be an idea to change that somehow too. You also need a whole new rear panel (the grey thing), plus the numberplate light is different. Old parts like these you might find in a junk yard for $100 if you're lucky, otherwise it will cost big bucks.

I'm just going on some assumptions and guesses here, I'm sure other (more knowledgable) people can help further.