You're right that it was before the 70s, and you're wrong about never. AJ Foyt, for instance, who never stepped in a Formula One car, has participated in several F1 races by virtue of his Indy 500s.
There is no one winner of the 24 Hours of Le Mans.
While there are different classes, there is an overall winner. Google Tom Kristensen, who holds the record for most victories at Le Mans. Or Graham Hill, who won Le Mans, a couple F1 championships, and Indy.
Could you give me a citation on when the Indianapolis 500 counted in the Formula One Championship? When was that?