Don't know what you are asking as you as it is difficult to follow your writing, but why would anyone need a V drive?
The engine can plug into the drive. Your asking why are the engines mounted rearward and the exhaust forward. Answer: drive line length. The engine is basically 48" long, the drive line would have to go this length plus. Also the engine would have to be mounted higher upward (when dealing with a Merc #6 that is). Then there is the cooling and such of the V drive which technically is a Parallel offset box. An actual V drive would put the engine at the typical 10 degree angle that is in the V drives.
Did I mention cost?