No, the Mutt tank is the same tank used on the Chopper and is 4 pt mount. The Mastiff tank is a 3 pt mounted tank. It'll fit if you want to cut off the mounts on the frame and weld on news ones to support the Mastiff tank.
Sounds like a good option. Much easier to re-work mounts then cutting a tank apart. Also makes it a summer project since I can ride while the new tank is out for paint.