1970coupe

well ive had my stang for about a year now and im still clueless to whats been done to the rearend and motor. is there anyway to figure out what gears are in the rear? i spun the tires the otherday in the street and it left 2 marks so im pretty sure the rearend has been messed with because i dont think the coupes ever came with posi. it used to have a 6c from reading the build sheetand now has a 351c so my best guess is that its been modified before in was stuffed in. anyway to tell? it looks to be stock. its pretty jumpy. maybe all the clevelands are that way tho?