More than likely You will have to take the heater box off and replace the copper rings ( gaskets) in the exhaust ports. They can be a bitch to get out sometimes so be careful not to damage the head.
      With the box off, take a good look at the surface of the 2 pipes that go up into head.  You may have to file them flat as they often burn a notch in the surface (common) and it will still leak no matter how tight you get it.
       Also be very careful with the mounting nuts. The studs can twist off in a heartbeat if you get heavy handed.   If you break one you don't know how much trouble you will be in.  
      It was a sorry,sorry way to install a heater box.  The copper rings are barely crushable if at all.
      The box may be just loose, but I really doubt it.
       Good luck