I agree with both of you about the LS swap. Technically it's the best, easiest, cheapest bang for your buck... which is why everyone does it.
On the other hand, everyone does it. And in the end you've got a Camaro-powered Z. Why not find a smashed up G35 and look into an engine swap from that?
Or if you want something radically different and you like to fabricate parts, a turbo I-4 from a Kawasaki ZX-14 would be impressive. Imagine a 250+hp 300ZX that shifts at 12,000 rpm!