We decided to go large with the smaller 70mm pulley right off the bat. We snugged up the pulley first.
Then installed the belt.
Then tightened down the pulley. As a side note, we moved the idler pulley slightly outboard on its bearing so the belt would be better centered on the idler. Scion does not really control this tolerance so some cars need to have this done if the belt is riding on the edge of the idler pulley. It is just a simple matter of putting the pulley on the bench and tapping it on the outer side of the bearing to move the pulley surface in relation to the bearing.
We prepped the intercooler heat exchanger brackets by putting all of the grommets and rubber pads on them before installing.
We mounted the heat exchanger in the front of the car. You can see how big it is here. It bolts in place with no fuss at all!