The 1968 S-code engine has a round vacuum port on the d/s of the engine and differs from the 289/ 302 air cleaner assembly. There is some confusion on the subject, due in part to cars with a Canadian DSO. S-code cars shipped to Cananda had the '67 air cleaner assembly with the screens in the front.
To further confuse the subject the X-code 390-2V had the same air cleaner as the 289/ 302 engines.
Here is a picture of the 289/302 Air Cleaner
Here is the S-code 390 Air Cleaner
Danny