From what I've read, a 120 isn't going to be able to house it forever. The fish will grow to 30" in length and will need a much bigger tank eventually. You also need a huge amount of filtration as they are very messy eaters. I've seen some kept as single specimens and others kept in community tanks. Apparently, some mbu puffers are very aggressive while others are really calm.
The 120 is too small for a mbu, as they can easily reach 24", which is just about the width of your tank. From what's I've read (and I know this is going to sound crazy) 1000 gallons is a minimum for this fish. They are simply too large and sensitive to water quality to house them in anything less. They're best saved for public aquariums.
There are some other great puffer species to look at though!
I once read on another website a guy purchased an MBU puffer from his LFS and they told him that his 5 foot X 2 foot tank would be enough to house him.
But before long the guy who bought the MBU found his puffer was out growing his tank when his puffer reached about a foot or more in length and decided to contact a specialist to find out more...
The specialist basically told him in order to house on these guys happily, it would be best to have a tank no less than 10 Ft x 3 Ft x 3 ft with lots of covering / decoration as well as subdued lighting.