>Does anyone know the heater tube size on a '68 bus - the single center tube. 
> From what I could see, it looks to be 3 inches.  I'm ordering a helper fan 
>from West Marine and they offer both 4 and 3 inch sizes.  Could a 4 be 
>adapted?  Might give even more flow.

I did exactly that and took their 4 inch inline fan. Also replaced the
slightly smaller heater tube with 4 ID aluminum ducting. Followed Tim
Smith's instructions and insulated the duct with fiberglass and duct tape.
I'm pretty sure that, with a little ingenuity, you can also keep your old
heater tube and adapt it to the 4 in. fan, if you want to.

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