Just looking up the feeding guidelines online and it says that a 60-80lb dog should get 4 1/4 to 5 1/4 cups of food per day. So 6 is probably way too much. You can reasonably cut that back to 5 cups per day and be safe. You want to be able to feel your dogs ribcage through its fur. If you can't feel ribs then the dog needs to lose weight.
There are a lot of foods out there that you'll have to feed less of. They might cost a little more but you save in the long run because your food lasts longer. Some examples are Taste of the Wild (3 cups per day), Wellness Supermax (2 1/2 to 3 cups per day), Canidae (2-3 cups per day) and so on.
Look at your dogs stools. Are they really large? You should be shooting for feeding the least amount of cups per day to keep your dog in decent shape weight wise, the dog should have a nice shiny coat with no dry skin and small stools.