Thats not really rigged up for back seats. Mostly those are more for the front seats. I mean i guess you could, but most people here i imagine are more worried about the front seats and leave the backseats just lapbelts.
I wouldn't drop that kind of money into the back seats unless you are a bad driver with kids in the back seat. Like it was already said, most people don't worry about the back seatbelts, they just use lap belts. Lap belts are around 25 bucks a peice from most of the mustang sales places around and they will be much easier to install. I will be putting a set of 99 cobra shoulder belts in the back but that is just because I got a good deal on them with the seats I put in my car. Unless you arent affraid to do some cutting on your classic, just use the lap belts.