the Shimano FD-7403 came in two flavors: FD-7403 B and FD-7403 F. One had the clamp (F), the other did not (B). I think that your problem might be that either a. the clamp (did you get an aftermarket clamp to put your braze on FD?) is too thin or b. the derailleur is mounted too high on the ST. I would suggest to trade your braze on derailleur for the one with the clamp, here on the ISO/for trade forum (I am sure that folks here have some of them sitting around) and take it from there...