The first issue is hub spacing; normal hub spacing for a 5 speed is 120mm and for a 6 speed is 126. However all is not lost. Suntour introduced a 6 speed freewheel named Ultra 6 that was designed to work with 120mm 5 speed hubs. So, similar to gyozadude, if you use a Shimano 7 speed shifter (they use the Suntour 5mm Ultra 6 standard) with a Suntour Ultra 6 freewheel, you can actually get 6 speeds and the extra click is usually eliminated by the derailleur limit adjustment. Since your talking legacy parts than you need to be careful as Shimano introduced reliable index shifting with six speeds in 1984 and later manufactured 5 speed indexing for cheaper bikes. What you need to look for is SIS printed somewhere on the rear derailleur or it may not be index compatible.