Place to send your original to if you don't want to buy a new one.

http://www.restorationradiators.com/

Detroit Muscle Technologies (Member 71TA on here) sells heater box rebuild kits.

One mine the white rope caulk in the rebuild kit was too thick to fit into the channels between the two halves so I rolled it down until it was about half the thickness that it comes in so as to not risk breaking the box when putting the metal clips back on.

Be gentle when removing the metal clips, and take pictures of it as you take it apart and then bag and tag everything.