I don't think it would matter what year the motor is. All Ford small blocks (excluding the 351's) from the 221, right up to the last pushrod 5.0 are the same size. The thing you need to know is the year of the belt drive you're using. If it's a '68 v-belt and '68 pulleys, then you need a belt for a '68.
The pulley layout goes from the power steering pump, over to the fan, down to the crank and back up to the power steering. The other belt goes from the alternator to the fan, down to the crank and back to the alt. There are no other pulleys or guides that the belt goes over or touches.
It doesn't matter. All you do is take a piece of string, run it around the pulleys the way the belt would be routed, measure the length, and add about an inch to compensate for the deeper seating of the string in the pulleys versus an actual v-belt. Then you go to a parts store and get a universal v-belt in that length.
All mid-to-late '60's small block Ford's used the same power steering setup, regardless of application. Some used different pumps, but all used the same bracket, so your chances of getting the wrong one are slim, assuming you tell them it's a (fill in the year) Mustang.
Replaced the belt today and not only did the squeal stop but I can honestly say the car steers a lot better. Not saying that it drives great or even good but it is noticeably better and reduced the dead feel when driving down the road.