The design of the OEM distributor hold down clamp leaves a lot to be desired IMO. There are a select few available that put a hold down foot on each side of the distributor body. MSD makes a good one and a few of the aftermarket Mustang vendors do also, I think it's a cheap insurance policy.
http://www.msdignition.com/product.a...utor+hold+down
What would cause the distributor body to turn? There isn't any torque on it? (Is there?)