Charles ,I am not sure if the index was used on regular Mustang or not in 68 ....
Might be getting off topic since were veering away from the S code application the OP was asking about. There are a number of small block examples in the unrestored picture thread examples of those.
- 68 Metuchen November Built J code Convertible Unrestored - Picture Thread
- Mid-year 68 San Jose Fastback - Unrestored Picture Thread
- NJ J Code Fastback - AC, PS, C4, Non-fold down - Unrestored Picture Thread
- 68 San Jose 302- Early December 67 build - Unrestored Picture Thread