First off a LT1 is probably going to be a cheaper and simpler swap, however if you are good as wiring and can find a cheap LS1 then by all means go for it!
I think a Vette LT1 has a different config. with the alternator, AC pump, etc. and causes some fitment problems. I forget which rear end came in the 260z's but your best bet is the R200. If you can get under the car look at the outputs on the diff. If there are five bolts arround each side and has a taller rear cover then it's a R180. If it has no bolts arround the output and has a round-ish rear cover,then it's a R200. Don't let the rear end stop you from buying the car, though. If it has straight frame rails and little rust then go for it. Check the floor pans, frame rails, 'dog legs' behind the doors and any other suspect areas.
Also, don't be afraid to search
Edit: Man, I need to type faster! There were no responses when I started!