I only think you will be able to use the dash and the rear seat from a 4 door to a 2 door.
The door panels will not work the window trim on the inside will not work the front seat will not work as it does not fold and the back glass is different.
That is about it as it is different in every 4 door I have seen to a 2 door.
On the out side the front clip will work but it depends if the 4 door is a Caprice and has the round head light header panel as from what I learned as the front fenders and hood are the same just the header panel has different lights.
on the rear the tail lights will work if they are the same looking and the rear bumper will work aswell.
The front bumper will only work if the 75 is again a Caprice.
Hope this helps.
Also go to the GlassHouse Fest found in post your rides you will get answers there aswell.