thanks. so this will work as a replacement for the shimano 600. whats i really don't know much about the mechanics and don't wanna buy something that doesnt work. the rear cassette i have is an 8 speed shimano hyperglide i believe its call.
Rear derailleurs are pretty interchangeable, especially for 8 spds in the back. Depending on whether you have a triple or not (I'm assuming not, as it's an older derailleur), you may need a longer cage. Also, many new derailleurs are set up top-normal, so they shift backwards. In other words, they're at rest on your large cog and shift toward the small one.