While the engine was out, I shopped around for a pair of used, but good,
heater boxes. The 2 pairs I saw were in worse shape than my own and both
merchants wanted $75 per box! That's the rustbelt for ya.

So I decided to fix mine. Cleaned them up in hot, soapy, water and let
them dry for a couple of days. Then painted them with POR-15 (paint over
rust), which gives the metal an almost ceramic-like, hard, finish.
Finally, I band-aided perforations with aluminum-foil tape, and voila. 

I'm sure there are more professional approaches, like welding. But I'm
just a regular guy (who can't weld), with limited financial and
handcrafting resources. So this is it. Btw, the Al-foil can also be used
to seal the gaps between the heater boxes and the tubes and flaps, etc.,
that make up the whole heating system.

I noticed a considerable improvement in heating output while driving
around in close to freezing weather.

-- 
Sami ('75 Westf.)
dakhlia@wuecona.wustl.edu
http://wuecon.wustl.edu/~samid/