Consult Sheldon Brown. The spacing between cogs on 6 and 7 spd freewheels and cassettes are pretty much the same within the same brand. If you try to mix Shimano and Suntour you'll run into issues, though. I've tried in the past. A lot of old indexed shifters are compatible with both 6 and 7 spd freewheels and cassettes. The limit screws on your derailer will define the range of motion that the shifters are allowed to operate within. A lot of old indexed shifters will have that hidden click at the end that will allow you to index 7 speeds. It's like finding that super secret hidden song at the end of the CD that isn't listed on the back of the CD case.