Things to consider.
1: cost. Is importing a motor that may only add 1-200cc's over the stock motor (subtract from available L28) worth the substantial wad of cash it's going to take to get it here
2: parts. How much will you drive it? Can you afford to wait for parts
3: Power potential, For all this work you're going to fab, is it worth the extra work over say an L28 et for whatever reason you chose?
4: Weight. A 240Z is a light car, is balance and total weight worth the weight of an engine that, depending on choice, may be quite heavy (I'm looking at you toyota).
I took all of these into account and did the only logical choice in my mind... I got a Mercedes.
...But in all seriousness, as much as it pains me to admit it, I'd say 2JZ simply due to it's availability as a USDM motor.