Are you sure you have a 6-speed hub? (Measure the distance outside of cone to outside of cone on the hub or inside if dropout to inside of dropout. A 6-speed will measure 126 mm. A 5 speed will measure 120 mm.)

The SunTour New Winner Ultras used closer spaced cogs and a narrower chain so 6 cogs fit in the space of 5 and was used on 5-speed hubs. If this is the case, a 5-speed will simple screw on and be right. (You will certainly have to play with the limit screws a little, but this usually the case anytime you change FW models and should always be checked.)

Ben