Original Datsun 240Z parts are getting harder to find, and the prices are starting to hke up. I know people running all SR20, RB26, and LS motors in their 240Z's too. The SR motor is a really good balance for the car, but I remember it was a bit pricey to get it in there properly. While the LS and RB motors are perfect for power, it isn't going to be great as a street car anymore. 300 hp is definitely way more than plenty in the 240Z for street driving.
LS motors are just so easy to work with, and parts are readily available. The motor itself isn't too heavy as well. Best thing is, when something breaks, just go to autozone and pick up a replacement.
Oh, and the the most reliable RX7 I know is a LS swapped one
