[Racer x]

Good luck . Consistency is very important. And being a strong leader. The mouthing is a bad habit. Don't really react to it. This gives an attention reward. Keep your hands clear of the mouth. I started grabbing his front tooth and holding on to it. He does not like it and stoped mouthing. I can't recommend that.
My dog looks like a brindle Pressa Canario. He is about 100 lb and eats a lot. I can't place him and he is a very powerful . They say what a lion is to a cat. A mastif is to a dog. I take him to work with me every day. He sleeps on the bed with me and is with me all the time. I don't really ever stop training. He is a guard dog. So I have to look after him. As long as he thinks I am protecting him. Then everything will be OK.

Big working dogs are not really pets. They are a lot of work. You need to work them a couple times a day every day. You need a fenced yard with a six foot stockade fence. Children and other pets are OK but complicate things. Be careful selecting a mastiff or any dog. Do a ton of resetch.
The loyalty and devotion you get back from a dog like this is amazing though. And they are very entertaining with there personality.