Sure-Grip limited-slip differential

Getting the rest of the things I need to finish my project. Working on a 69 Dodge Dart Gt. Installing a 440 4-speed and the 8 3/4 one wheel peel that needs an upgrade. This has cost me a small fortune and at this point funds are low. Anyone have one for next to nothing this would be greatly appreciated. I am not looking for free, but reasonable.