IMHO, if the engine swap was done right and for legitimate reasons (defective engine parts), the effective value to me would go up, seeing as you're getting lower engine miles than the odometer reads. But I'd be super careful about researching the engine swap: get the paperwork from him for the warranty replacement, drop by the dealership and talk to the technician that did it about what the situation was, and go over the engine bay from above and underneath very carefully (with the plastic undertray removed) and double-check all of the hoses, electrical connections, etc.
The problem is, not already being super-familiar with the 370Z, you may not know what you're looking for. A test drive would tell you a lot too: that the engine's running smooth, the gauges all look reasonable, no CEL being thrown, no sounds of exhaust leaks or rattles from the engine bay area, etc.
But just the act of swapping in a new engine block, if done carefully and correctly, is really an upgrade. Brand new parts replacing used ones. The dealership dropped my engine out of the car to install my long-tube headers, and they put everything back correctly just fine. On the other hand, the particular dealership / mechanic I used is very reputable and capable (@ Baker Nissan here in Houston).
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