I would be cautious in going 100% poly. What you have is a 38 year old structure that has seen its fair share of transient loading and vibration. You might want to keep some rubber in key locations on all corners to allow some pliability. It seems you're building a street car, so I would be careful about stiffening the springs too much. Remember, the Z didn't have a Cadillac suspension to start with. Stock doesn't mean soft, it means a lot better than the worn out suspension you have now. Few people on this site even know what a stock Z rides like, including me. That being said, stock springs are not available from any store or website. We once had some made but to my knowledge they were all sold. You can check with Courtesy Nissan to see if they have any of the Euro springs left, that we had made as well; I don't know if they do. I'm not trying to push a stock setup for everyone, but there was a time where everyone was taking their beatup old Z from worn out to track stiff and lowered, and were later sorry they did. If you're racing and autocrossing a lot, there is no question what you should do. If your driving pothole ridden America, think twice before you make big leaps. Cars built today are far more rigid and heavier as a result. You can't treat a Z like those cars. You have a 2400 lb GT, not a Formula 1 racer. I'm building a road car for taking trips, etc.... so I installed new stock springs, stock-type shocks, some poly and an upgraded fr/rr anti-sway bar kit. You might be able to deflect a corner of my car by an inch if you lean on it. That's my 2 cents. Good luck.