Well it's definitely not an 8 3/4 with 3.73's. Rear cover that's oval shaped and has 5 bolts on the lower half screams 8 1/4. A 340 car would have come with an 8 3/4 in 1971 (I need to double check that though). So the car probably started out with a 318, which might still be in it. Seller claims 360 and 727, which is possible if it was swapped in but definitely not factory.
The only way to know what it is would be to see for yourself. Either the seller is an idiot and doesn't know what he's got, or he's trying to pass it off as something it isn't.
It could be an alright car but proceed with caution.