There are lots of choices here and the one you choose will determine what complications you have down the line. I have chosen to mate my motor up to the existing gearbox on a rear wheel drive car. That means everything from the gearbox back is standard, but I have to make up an adaptor for the flywheel and a plate to match up to the gearbox bellhousing.

I could have chosen instead (and might yet) to use the gearbox from a Lexus GS450H, which contains two electric motors and kicks out an apparent 300 horsepower. In this case, I would instead have to mate the back of this gearbox to the propshaft. Some people take out the clutch and bolt the electric motor straight to the input shaft. Some people do away with the gearbox altogether. It all comes down to your component choice, but every decision changes the complexity of the project down the line.