'67/68 rear is 2 inches wider but the mounting points are the same. (I have a '67 rear in the car in my sig-pic. Sharp eyes will see that the tires appear to fill-out the wheel well a little more... which looks better, imo.)
I'm running a 67-70 rear in my 65. As stated it is 1" wider on each side. It is a 9". I'm running 2004 mustang bullit wheels (17" x 8") that have 2" additional back spacing. It eliminates the need for wheel spacers with this combination.
Does your current rear have the fill plug in the back or on the side?? That can help clue you in as to what year the rear end is. Try also searching kevinstang I think it is spelled. He has a lot of useful info on his site for 8 and 9 inch rears. HTH