Flywheel Upgrade from 157 to 164 tooth?
From my experience, I would say that most early V8 Mustangs with the small aluminum bellhousing came with 160 tooth flywheels and flexplates. The 157 tooth type is interchageable with the 160 tooth variety. I'm not convinced that the starter drives are interchageable between the small and large flywheels, but I can't say for sure. There are some funny matchups with those combinations. As an alternative, you may want to consider what I did with the Fastback. I used a 5.0 type billet 157 tooth flywheel that was rebalanced to 28 oz. This allowed me to use the 289 small aluminum bell, and the late model 5.0 "dowel located" 10.5 Centerforce DF diaphram clutch. This makes a smaller package and less weight (static and inertial) than the larger components. I don't think gripping strength is an issue.
66 Fastback restomod (12.8 @ 112 mph)
85 GT (preserved)