Well first off is your 400 Suntour or Shimano equipped? If you have Suntour you made to upgrade everything. If you have Shimano you may be able to use a newer wheelset with a 7spd cassette and 4.5mm spacer in back. Then just get a 7spd shifter.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk