[color=red]1-I’m talking about the ABC studios in their heyday of epic movies.
I suppose it could mean that their heyday was when they were making epic movies.
It could also mean, I suppose, that they had a few heydays and this one was the “epic movie heyday” (as opposed to, for instance, their “western heyday”). But I am not sure one could have more than one “heyday”.
In either case I think “epic movie heyday” would work better.
As you suspect, ‘in their heyday of making epic movies’ makes more sense. However, usually ‘in their heyday’ would be enough. After all, what would a movie studio do other than make movies?