Stock Rear End Size and Gear Ration in 71 Demon 340

71 340 car would have an 8 3/4.
As for gear ratio, i would bet on a 3.23 open being the base gear, with 3.55 and 3.91 optional, along with suregrip.
Happy to be corrected by somebody that knows for sure.
Easy enough to find out what you have. You'd need a couple pieces of tape, and a floorjack (and jack stands!).