My 79 Chrysler 300 has a 9 1/4 3.21 rear. Not long ago Mopar Action did a write up on a suregrip install. They had good comments about it being a stout rear. I think gear ratios are up to 4.56.
Full blown retail with a guarantee at a salvage in Georgia is $200. There is one in Kentucky for $100. There is plenty around the country for $100 to $150 and some as low as $85.