motor mounts are identical. its about money. Nissan wont spend the money to make a completely different chassis just for a 4cyl. or 6cyl. engine/
but yes, ecm, harnesses etc will need to be swapped as well. it'll be cheaper than buying a 3.5, but the time involved makes it well worth your time to jsut invest in the new car. and if you are going to pay someone to do the swap then you or certain to save time and money by getting a new car, unless you get everything needed for a swap from a junked/wrecked 3.5
there's many options, but its up to how much time and money you're willing to invest.