I've got JCWhitney's $70 fan in the main heat duct on my 81 Vanagon.  It helps
a lot, although it is very noisy.  If I could do it again I would consider
mounting it in the engine compartment.  This type of fan, together with 
scrupulous care to make sure your heating system is tight from end to end (e.g.
high temp red silicone sealing the heat exchanger case to the pipes), will do
a lot.  Enough for C-c-c-canada? I don't know.

Make me an offer if you want my VW gas heater.  I don't use it enough to
justify having it under there losing value.
(NY/NJ metro area)

B.