a qualified , in person dog trainer would be your absolute best bet , big dogs like you describe can be pretty dangerous …... i'm not a dog trainer , but I've trained a lot of dogs … my approach wouldn't work for you , but I've seen it done a way you could probably go , a muzzle , ignore , distract , move on and repeat when neccessary……. ignore the muzzled dogs threats , get the dogs attention onto something else, don't let him even think he's getting rewarded in getting his attention changed , just help him put his attention elsewhere without him knowing it , walk away and do something , it'll probably distract him , the person he threatens needs to not react in anyway ….. when his attention has been distracted to something else go on about normal behavior , you could get him to do something for you or possibly the person he threatened , as long as he can't relate the reward to the threatening behavior in any way ...….when he gets nothing out of the behavior he'll eventually give it up ...….as for people touching him , don't allow it unless he's accepted them ……. getting him to accept folks can be a job in itself , requires desensitization , again best done by a qualified dog trainer , it's doable …… some dogs just don't like strangers , they can be taught to tolerate , personally i'd just not allow folks to touch a dog that's just tolerating it …...jmo …….