so you are talking about the AAC then, not the IAC ?
2 plugged unit - from looking at another RB20 plenum i've worked on - the AAC can be installed by simply drilling 5 holes in the flat back section of the plenum - 4 bolts holes and the AAC's plenum feed hole.
hardest thing about it is :
- having a enough room to fit it
- flat surface for mounting
- running the air hose from the IC piping from infront of the TB
so its not that hard, just a little fiddly.