Here is a way to ID your rear end. To see if it is a sure grip or not. Jack up the back of the car then turn one tire. If the other tire turns the same way it is a sure grip. If it turns the opposite way then it is an open rear.
A 340 car wouldn't have come with a small rear end. It has the 8 3/4. Was the small rear end available in an E-body in 72 with the /6? I know by 73, all rear ends in the E-bodies were 8 3/4.
You could still get an 8 1/4 in a B-body in 73 though.
Would not hurt to check. After 40 years or so you never know what some of the previous owners have done to a car. The question was not should it have a 8 3/4 it was does it have an 8 3/4. Now he can check and know for sure.