I have had bad luck with rebuilt afms. I do not believe it is necessarily a part that needs to be purchased rebuilt, like brake pads or spark plugs. Get one from a junkyard and save 150 dollars. Most of these parts stores will only give you a year of warranty anyway. It either works or it doesn't-at least, that is what I was trying to find out from this post(?) If you are not getting an afm code on the ecu, then there is nothing wrong with it or the harness.
Did you ever think about the possibility of removing the AFM and not replacing it? My friend did that with his '87 Mazda RX-7, and the car seems to run better without it. Of course, he had to block it off where he removed it. Is that a possibility for the 300ZX, since that's just emissions hardware anyway?