Does the center chunk come out from the front? Or does it have a rear cover to access the gears? Start with the wheel bolt pattern. I assume it's a 5-bolt pattern. If the chunk (hog's head; 3rd member; whichever you are accustomed to calling it) comes out the front and it has a 4 3/4" wheel bolt circle, it is a '55 to '64 Chevy rear. If the chunk comes out the front and it has a 4 1/2" wheel bolt circle, it could be either a Ford or Chrysler. If the chunk comes out the front and it has a 5" bolt pattern, it could be early Olds or Pontiac or 9" Ford. If it has a rear cover and 5" pattern, it could be Ford or GM; 4 3/4" pattern, it's intermediate or metric GM, 4 1/2" bolt pattern, it could be Ford or Chrysler. This is the tip of the iceberg; there are infinite possibilities. Post a pic; that will help...

One last thing - in the early 80's, the '55 - '57 Chevy and '67 & up Camaro rear ends were popular swaps because they had leaf-spring pads on them and were almost a bolt-in swap.