Ran into a problem with my install. The forward end of the spacer is touching a screw on the alternator, and almost touches the alternator heat sink.
The the original plenum cover has a flange a bit larger at the mating surface. The spacer is designed to match the flange. The original plenum has an area milled to clear the alternator. When you insert the spacer that moves the larger area outward to the right, so the original area cleared for the alternator is now in the wrong place.
I am having a local machine shop mill off a little material off both the spacer and the original flange. About 1/8" to 3/16" should be sufficient and there is plenty of material remaining.
No one else has mentioned this so it might be a variance from car to car. I don't think my alternator has ever been off the car. Not hard to correct but definitely inspect during your install, you would not want the plenum to actually touch the alternator.