What's the best car in the list?
The Mustang would probably be more reliable, and parts will be cheaper and more plentiful, unless you're in Japan (in which case they might still be cheaper). It also has a back seat.
The Datsun will handle better by a wide margin, and, IMO, look far better. (but you might like the Mustang better) Parts will be considerably more difficult to come by, particularly any 260Z-specific parts (240 and 280 parts are more plentiful...the 260s are sort of the orphan Zs) Also, if you don't make any modifications, the Mustang is probably faster.
If you get the LeMans, I think NOT turning it into a GTO clone would be cooler than doing so.