Round or oval in shape? If it's an oval shape, then it would be the '55-'64 OldSub mentions. If it's round, then that would be Olds or Pontiac. If it is a Chevy, parts are relatively easy to get. I find 3rd members for them all the time, many with Posi. Also, 'Vette diffs up to '62 will swap in as well, opening up even more options for gearing/posi untis.