I recommend against making any such radical changes to any modern car. The days are gone where you can drop in any engine of your choice. You'll need all the plumbing and wiring from the donor car. All the accessories attached to the front of the engine will be in a different location. And you're sure to run into a number of wires and other connections that just won't fit together. If not every electrical connection is perfectly made, you're sure to have a check engine light on all the time which means you won't be able to do any meaningful on-board diagnostics, which in turn means any hiccup with the engine will have to be blindly dealt with. Even if all electrical connections are the same, internal calibration and engine parameters would probably be different. Few people care for modified vehicles should the time come to sell the car.
It's just not worth it. Stick with original. If you want a V6 engine, sell what you have and buy a car made with a V6. It would be a lot easier in the long run.
95 E320 Cabriolet, 131K