What size clutch in a 65 Barracuda Formula S ??

Yep. The scalloped pressure plate uses socket head cap screws. You use an Allen wrench or hex driver and not a wrench. They must have small heads to fit and work properly. (you know but many of the readers here may not)

the bolt must also be shouldered to fit the pressure plate holes. standard threaded aren't close enough tolerance and will cause vibration or shear off.

fwiw, the bolts i'm running aren't socket head, they're hex head cap screw like 3/8 head.