About that leaky exaust...I have one of the '70 vans.  I believe the engine
of the '71 changed to a 1700 or something, but the heat exchangers are
probably similar.  I swapped heat exchangers when rebuilding and found that
the heater cables wouldn't reach.   So now I just wire the levers ON in
winter and undo them in summer.  If you disconnect the cables at the exchanger
make sure you don't lose the litte connectors that hold them;  they will fall
right out.  If that doesn't fix the exaust problem, your heat exchanger flaps
are not working properly;  the spring that holds them closed is broken or
somehow disconnected.  Heat exchangers can be taken off and fixed without
engine removal. That's the best quick and dirty fix.  DO NOT recommend blocking
off the air going thru the exchangers. That will cause the engine to run hot as
a firecracker because  heat rises to the valve heads and could warp them or
destroy the rocker arm cover gaskets, causing the oil to run out...NOT GOOD.
There are other things that could cause exaust problems, like a poor seal
around the engine.  Exaust will inevitably be pulled back thru the fan and
into the heater system.  An engine compartment seal could be a cheap solution.