I'm not sure about the 1960s, but the 1970s front derailleur that was typically paired with the Allvit rear derailleur was the model 700. It was designed to be compatible with both double and triple cranksets.
Schwinn used the Huret 600 with the Huret triples on the early to mid sixties Super Continental, Sierra, and Superior models.
It has enough lateral travel, but not sure what the max difference between ring sizes is for them. The "granny" ring on these was 40 teeth. I've replaced the 40 with a 36 on one of mine and the derailleur will still handle it without the chain dragging on the rear of the cage, but I don't think you could go much smaller than that, at least without also going to a smaller large ring.
Thanks. The only 700 I found on ebay was a "Schwinn Approved". I'll try what I have first and move on if required. Finally got the crank to fit after trying 4 different spindles. I have lots of all Campagnolo bikes, starting to build them less high end.