I once had a dog who was also deprived by his previous owner and had the same dilemma of constant barking day and night. My Vet adviced me not to shout and yell on him for to him it means I am joining him, me as if barking also by yelling at him.
I bought a dog treat and breaks it into smaller pieces. And whenever he start barking again, I signalled with the Shhhhhhhhh and say in a soft but commanding tone 'Quiet.' Once he stop barking and gets somber and quiet, I give him some of the piece of treat I had.
Its a routine and might have to be repeated several times until he learns about the command.
On Dog Treats: there are many available in the pet shop but I chose the one that resembles a dog bone, hard, and long lasting. It takes days for him to nibble on it and just re heat it on the microwave every time I need to treat him or reward him.

The important thing is that your patience, training and reward. They all go together most of the time.