Rear derailleur help

I am trying to change the gearing on a beloved seventies 6/7 speed. I would like to use a 50-34 compact crankset but stick with a 6 or 7 speed freewheel in back so I don't have to change wheels and spread the frame. I need rear derailleur ideas that could handle a 30T capacity and a 28T rear cog. Do you have any suggestions? A website suggested the Campagnolo Veloce 10 speed derailleur, but when I tried this model, it doesn't work, even with substituting longer adjustment screws there was too much travel toward the spokes. Thanks.