^^ What DJ Ray said; a swap between drivetrains will mean at the very least the electronics (wiring loom, sensors, and ECU, as well as possibly the alternator, A/C compressor, etc) between the two engines would need to also be swapped, as well as engine mounts, and as he suggested, maybe even the subframe, springs, brakes, and so on. Even accessories like the A/C compressor and alternator might need swapped. It's likely you would need to swap transmissions as well; even if the 4 and 6 cyls use the same transmission (idk), it's not likely they have the same final drive ratio, so it would be likely the original would have the v6 spinning needlessly fast, especially in top gear. For this to be feasible and affordable, you'd be best to find a good-condition insurance write-off that got hit at the back to use as a donor car, so you could get all the bits you need off of it.
I did a swap something like this, between a 95 and 96 Passat VR6 - my focus was more to get the good manual gearbox and clutch bits out of the wrecked, but running and driveable 95 into a low-miles 96 with a very expensive-to-repair slushbox that I had picked up for a song. Even though the engines were nominally the same, and I didn't have to swap or even depressurize the AC bits, I still had to make adaptations to the wiring to trick the car into thinking it was in park so it could start,, wire in the reverse lights, and so on (and strip out a big loom and separate powertrain control computer that ran the autobox from under the rear seat).
I figure it took me about 120 hours of work to get it all done (which did include some extra swapping of nicer interior bits, plus my Koni yellows and some other good suspension bits from the donor car, and was done in Dec-January in an unheated garage in Canadian winter, so it was fricking cold (a lot of days in the -15C to -30C range), which added time). It only made sense because I was unemployed, and therefore had time, but not a lot of money on my hands, and am pretty mechanically handy. Because the manual transission was a popular swap, I also had forum help from VWVortex to guide me through the electrical - if this swap isn't covered here or elsewhere, you'll get to discover any issues in terms of things that might keep the swapped drivetrain from talking to the ECU and, in newer cars, the body controller and security electronics, to meet all conditions needed to start and run.
In other words, these sorts of things can take a lot more time than you might guess, and you need to be prepared for more than just the mechanical basics).