A lot of ganaches are made with 1 pound of dark chocolate 1 cup of heavy cream and 2-4 tablespoons of butter. I think that would be too heavy for your application. I recommend using 1.5 or even better 2 cups cream. That will make it light enough for donut fillings.
Whipping it once it gets down to 85°F or so will make it lighter still. You will still have lots of rich chocolate taste. Whipping introduces air into the ganache making it lighter.
What is more typical in donuts is a chocolate pastry cream which is very similar to pudding. You might consider that option, too.