Ruining the car? Why would anyone think that swapping in one of the best engines ever created was a bad idea? Well...so long as it's a clean install and not some garage-mechanic, hacked up bastardization
LS RX-7s are some of the coolest cars around. The FD was an
amazing car with a sorry excuse for an engine and turbo-system, so ditching that for an LS-block makes perfect sense. What you end up with is a badass street-cars combining one of the best sports-car chassis of all time with a light, efficient, and powerful V8. There was a very,
very clean FD for sale around here not too long ago with an LS7, and the car overall was amazing inside and out.
An LSx is great for an old 350Z that you can find for real cheap and end up with a really nice project. However, I don't think the hassle of swapping anything into a 370Z would be worth it at this point. As cool as a 500+ RWHP NA V8 370Z would be, you can just save yourself the hassle, headache, money, and significant downtime and just buy a Z06.