My '75 280z originally came without an overflow tank; just a hose that ran the overflow to the ground.
The overflow tank that was put on by a previous owner is an old 12oz. Pepsi can - pull tab type, not the kind with the tab attached.
I had the same questions about the differential vibration dampener. It's been discussed thoroughly over on classiczclub here: http://www.classiczcars.com/forums/showthread.php?t=25827
The stock steering wheel is 15 inches (~380mm) diameter.
Not only is a smaller wheel more difficult to turn - especially at slower speeds - the smaller diameter wheel will also block your view of the speed/tach gauges.