Keith Bluemel's Book Ferrari Original V8 says I think on page 36 that rustproofing began on all cars made in January 1984 (he goes through describing the rust proof process that was used). This book is a great resource for 308/328 owners, if you do not already have it you should order it. He does not mention a starting VIN. But if you are looking at US spec cars check the door plate and you will know when the car was made and if it has rustproofing. I do not believe Euro spec cars say when the car was made on the door plate.
1989 328GTS Red/Pelle Rosso & Crema Interior