I have done this also... Unfortunately, in the rust belt, with road
salt and so on, the long term results are not terribly good.  Salt
gets in between Aluminum and Steel, forms a nice battery of sorts,
slowly corrodes the steel.  Make sure you pressure-wash the underbody
a couple times a year... 

Good Luck,
Cetin

Sami Dakhlia writes:
 > 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/