Tough brisket

Make sure it's tightly covered while baking, and add some liquid to the bottom, such as water or broth to keep it moist. I prefer poaching it in a slow cooker, but you could do the same thing in a Dutch oven in the oven or on the stovetop.

I agree with loeello that you may well need to cook it for an additional hour or so. Also make sure you are slicing it across the grain instead of parallel to it.

I'd think too chewy means that it is not cooked long enough. What about cooking it further, like around another 1 hr (to 1.5 hr) at 325?