Many years ago I had a '69 bus, whose original heat exchangers were
wonderful. On a zero-degree Colorado night, fire the bus up, get on the
freeway, and ten minutes later you could take off your jacket and drive
in shirtsleeves.

Finally one heat exchanger developed an exhaust leak, so I replaced both
with inexpensive replacements, aftermarket types I guess. (Not J-tubes).
The heat output of the new ones was only a fraction of the old ones-- I was
_very_ disappointed. I don't know what the differences were, but they had
to be significant. Unfortunately, I also don't know what to look for when
shopping for them now, which I have to do for my recently-purchased '71 bus.
I'm considering J-tubes and a gas heater, just to avoid a recurrence of the 
past disappointment.

The moral of the story is, if you buy new heat exchangers, make sure
(somehow...?) that they are identical to the originals, in airflow, metal
composition, number of internal fins, etc.

Good luck!

-Steve Maher         smaher@ball.com         '71 Transporter