Indeed you Suntour 6sp Ultra freewheel has 5mm spacing and a Standard or Accushift 6sp of the day has 5.5mm spacing. Sheldon Brown
http://sheldonbrown.com/cribsheet-spacing.html Also you need to make sure that the rear shifter and derailleur are both 3000 as 3000 system is different from the rest of the Suntour line. If your freewheel doesn't have to be period specific, the current Shimano 6 speed freewheels are spaced at 5.5mm and they shift really nice!
http://www.amazon.com/Shimano-6-Spee...peed+freewheel