If you use a 7 speed freewheel you'll have to use the friction mode as your 6 speed will Not click 7 speed.
Also if you use a vintage shimano 7 speed freewheel you'll have no problems, however, if you use a new shimano 7 speed freewheel with the rivets on the back of the Low sprocket you will likely need a spacer in order for the small sprocket to clear the frame.
Lastly if you decide to go 7 speed SiS click shifting, you can go 105, 600, or Dura Ace 7 speed shifters and it will work just fine with that 105 rear derailleur.
Last edited by texaspandj; 03-30-20 at 05:27 AM.