Can you post a picture of the cassette? is it cassette or freewheel? What brand it is? Shimano?

For freewheel shimano is this one http://www.amazon.com/Park-Tool-Free.../dp/B001B6RGXG

If it's a cassette from that era most likely is uniglide where the last cog is threaded (normal thread) and locks the rest of the cassette - for this you need two chain whips to undo the last cog.

Check this link to identify what your cassette looks like and what tool you need http://www.parktool.com/blog/repair-...ewheel-removal