OD tranny hard to come by?

I am also interested in an a500 in my Dart. I calculated that with 24.5 inch tires and a 3.91 ratio out back that I would effectively cut the gear ratio down to a 2.50 for highway cruising. Unfortunately for now I can't afford the 2k for a 500 ready to go,....and I would like to see if one would go in an A boy without cutting the tunnel as my car is unmolested and would pretty much like to keep it that way.