Vgon diesel engine is longitudinally mounted, and trans is Vgon-like.  I
don't know of any VW or Porsche rear-mounted engine that was anything but
longitudinal.  Kennedy Engineered Products (no relation) has lots of 
adapters, and VW and Porsche aircooled engines have had the same four
bolts on the same bolt circle as their engine-trans mount since about 
forever.  I expect that a rabbit engine in your 914 will protrude into
the cabin unless you lengthen the wheelbase.  Pancake Type 1 (ie Type 3)
engine in 914 might make a good mileage competitor, but as an engineering
exercise rather than a fun car.  I would never stomp on "what ifs"; I just
keep getting stuck on "what if I had some more money."
B.