Clamping the cable over the bolt doesn't do any good, still not enough travel.
Anyway I found some pics from a 600 catalog that show the ST-6400 alongside the FD-6401, not the FD-6400 (which is the one I have). I'll presume the derailer is not technically compatible with my levers, even if there are ways to force them to more or less cooperate.
I just ordered a FD-6500 on eBay, I'll check back in here when it shows up to see whether it works or not.