Rear end ID

Could be an 8 3/4 or a 7 1/4, 318 cars came equipped both ways. In '71 there's a good chance it's an 8 3/4 car, but no guarantee.

Check the guide posted above, easiest way to identify the 8 3/4 is the removable 3rd member and the smooth round back cover.

Either way, there should be a small metal tag on one of the bolts, either on the 3rd member or the back cover, that should tell you what the gear ratio is if it's still there.