[HawgRyder]

If I were forced to use 12 cylinders in your situation...
I would think about stacking 2 911 engines end to end (both in the same orientation).
That means the flywheel of the front engine (remember..this is a mid engine car) would be connected to the front snout shaft of the rear engine with a coupler of some kind.
This could also afford you the opportunity to place a bevel gear system on the coupler to get a horizontal fan installed.
Some sheet metal to cover the gap...and a support for the fan unit...a BIG air shroud...and separate oiling systems.
Lots of work.
A long time back we did this with 2 VW engines to create an 8 cylinder race engine...mixed results...it was heavy...had vibration problems...and was a ***** to tune.
Bob