Before you do anything, post a picture of the cowl tag. There are literally thousands of fake 69 z28's out there so you don't want to get taken by one.
Also, post the VIN. It may already be in the database.
The original engine for a Z28 would have been the 302. The transmission would either be a Muncie M20, M21 or M22 4 speed depending on the rear end gear ratio. As for the rear, it could be anywhere from a 3.07 to a 4.88.
Ed