I always multiply by 1.5, 2.0, or 3.0 times the amount of labor Nissan tells me, specially when I am doing some work that I have never done before and based on the complexity. For example, my first struts replacement took 3.0 hours. Now, I do it in 30 mins per side.
Just make sure you have the service manual handy and review the procedure several times before starting the job. You can save that labor but you still have to pay for refill the system one more time.
I would not change the heater core if you used to flush the coolant regularly to keep the internals free of corrosion.