As Bill J indicates Daytona and Sebring were part of the FIA World Sportscar Championship in 1966, therefore FIA rules applied - suit box cases would be required.
https://en.wikipedia.org/wiki/1966_World_Sportscar_Championship
Daytona & Sebring were also part of the Championship in 1967.
https://en.wikipedia.org/wiki/1967_World_Sportscar_Championship