I'm Sorry, but I believe I have some bad news for you.
You are going to have to get a custom intake manifold or custom M90-to-Series-1-L67 intake manifold adapter made to put a M90 on an L27.
To date we, here at BonnevilleClub are unaware of anyone who has put a M90 on a Series I 3800.
It would be a very difficult task considering the length of the engine deck and the M90 -especially with the extension piece for M112 internals -the SC snout and pulley will practically be 6 inches (15MM) past the rest of the pulley'* on the engine.
This is just a start of why it won't fit (easily).
Your best bet is to get a M62 supercharger, S1 L67 lower intake manifold, and all necessary pulley'*/tensioners, a Mini-AFC, and scan the car to find out ho much KR you are getting.
From there get a smaller pulley, Series I L67 injectors and enjoy boosted L27 power with your RWD.