For the diff, turn both output shafts at the same time, and count how many times the input shaft turns for every full turn of the output shafts. That should be your ratio. If it's the stock R180, I believe it should be a 3.36:1 ratio.
For the mirrors, I'm just guessing which ones you have, but they sell replicas on eBay in pairs. They're not an exact copy, but close enough.
http://www.ebay.com/itm/DATSUN-240Z-260Z-280Z-CHROME-PAIR-FENDER-DOOR-MIRROR-/310353607742?pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr&hash=item4842843c3e
As for your intake, I don't get why you want to put it under the cowl. I've seen it done once, and that's because the guy installed a VQ35, and the intake points backwards on those, so he didn't have much choice.