Well it was replaced. If you can see the casting numbers on the heads and intake post what they are. On the intake it will be on one of the front runners. On the heads between the center 2 spark plugs. After 30-40 years just about anything could be there. Even a 360 as they used the same block as the 390 just a shorter stroke. Once you measure the crank stroke you'll have a better idea except for the cyl bore.