741 Rear

Would my 70 340 Duster have come from the factory with a 741 Rear? According to the fender tag it came with a Heavy Duty 3.91 rear. Isn't the 741 the weakest of the 3? I haven't checked to make sure the 3.91's still exist but from what I felt when I drove the car they or something close are in there now. I just lifted the car last night to check the ratio the old school way but didn't finish. I will later.