Stock Rear End Size and Gear Ration in 71 Demon 340

I would assume an 8 3/4

As for gear ratio. Look in yoyr factory service manual so e times they list what was stock for a particular application

Do you have a fender tag or a build sheet?

There are options that could have been ordered for gear ratio, like a towing package might have a diferent ratio, a limited slip might have another ratio