I'm guessing the idler needs to be in between the generator and PS pump, but closer to the block. Not sure if there is room for a spring loaded one in there, but that would be nice.
Ideally the idler would be mounted as part of the bracket you build to hold both the generator/alternator and the PS pump.
The idler I used was the cheapest one I could find at the auto parts store, it is also readily available and about the size I needed. Then it's easy to get and replace if needed.
Grigg