I have a 1970s Schwinn Continental with a 14-28 rear cluster. (Front chainwheels are 52 tooth and 39 tooth). I'd like to put a 13-34 7-speed freewheel on it. I know I'll have to re-set the distance between the rear dropouts and get a longer axle and skewer.
I'll also need a new (or used) rear derailleur that can (a) pivot front-to-back enough to handle a shift from 52 front/34 rear to 39 front/13 rear while keeping tension on the chain, and (b) move right-to-left enough to cover all seven cogs on the freewheel.
Can anyone recommend a decent (hopefully cheap) rear derailleur that will handle both (a) and (b) above?