measure the axle tubes or space between the U bolt legs, 2.5" it's a 7.25, 3", it's an 8.25
cover shape of the 7.25 changed in the early 80's, and the tubes grew to 3" in M bodies, but it necked down to 2.5" at the center casting.
never seen a 3.31 ratio tag before, I was wondering it someone...