I've got a long run that's flow insufficient: I bought a booster fan but need to know if I should mount it close to the plenum or further down the duct? (8" duct, in-line)
Wiring: the Printed Circuit board has terminals to connect additional features such as humidifiers, but in my mind you shouldn't place that kind of load on the card, more like using it to signal a relay to run the booster fan.
I'm competent enough to have installed the entire heating & cooling system (dual forced air furnaces & A/C) but this isn't my normal kind of work.  

I'd put it part way down the branch line personally.  But I have no professional justification to back it.

8" seems rather small, but you still need to provide more specifics.  How long is the duct run?  How many tons is the furnace/air handler?

If it is flex duct make sure it is straight and stretched out as possible with no sags or unnecessary bends.  If it is sheet metal make sure there are no leaks, seal it up.  2/3 of the way from the system or whatever the instructions say.