It will be a major task for a nonbodyman , but if you have the tools,the garage,the time and the patience than it is doable. I would start by searching on this site and others( maybe zcar.com) and read up on body work and rust repair . especially body work on your particular year and model z. I have spent literally hundreds of hours researching rust repair before I take on mine this summer.Going to a junkyard and cutting off the back half of a car in 10 minutes is totally unrealistic. Plan on the better part of a day. Take your time and cut the donor car part bigger than you plan on needing. You can trim it down later after you get it back to your garage. Plan on taking the entire back of the car and not just the skin. You never know how far the rust may have gone where you can't see it. If you don't need the entire back you can always pitch what is left over. But better to have it when you need it than to have to go back to the junkyard for another one. Take your time and seek advice if you are not sure and you'll be able to do it, but plan on it taking 10 times longer and 10 times more expensive than you think it will.