If you're riding a non-indexed system, there are zero compatibility issues (aaah! the good old days), except possibly width of the cage at the pulleys. Any long cage 6s, and most 7s and 8s RD will work no problem. You might find a good deal on a suitable one here. If you're unsure they also answer heir phone and offer sound reliable advice. You can tell them the Chain-L guy sent you and they won't charge extra.