I have a Dura Ace freewheel with the same body design. Nothing special about the DA or the 600, fit and function-wise. You can replace them with any standard spaced 6-sp FW. The one thing to be aware of is the removal tool is specific to this body design, AFAIK. A Suntour 2-prong tool will work, but you have to use it judiciously.
If you decide to chuck the 600 FW, drop me a line. Maybe we can make a swap (for something more conventional going your direction).