I was wondering that can i install those strut tops which are made for car built before 1985 to after 1985 model? I have that kind of itch that they are interchangeable.

Are you refering to the early ones that have metal around the top? If so they fit exactly the same.

Yes. Because newer ones aren't available anymore from main dealer but there is a change that main dealer still sells the earlier model.
So if i ordered pair of those older ones is it a strait swap or is there something extra needed?

If the early ones are still available that is great as they seem to be better. Should bolt straight on as I've seen cars with a mix