Why dont you give Rob Tuckett from Tucketts Tyrepower in Hastings a call. He is in the mustang owners club, is very experienced racing an early stang and often helps members with tyre choices. Looking at the site it appears there a far more options for 195/65-15s But talk to Rob
Think I would go 195/70x15 . Side wall is a bit taller for a better ride . Still only 25.7 inches tall . The 65 is 25 inches tall