My bike is a 1986 Centurion Dave Scott Ironman with 600ex 6spd 13-21 cluster. My 60 yr old legs cannot get up some of the climbs anymore with nothing lower than 42/21. I have read in a couple places including Sheldon Brown that I should be able to switch to a 7 speed freewheel (thinking 14-28), new chain and index shifting will still work? Just making sure before I order parts. Any and all ideas are appreciated!