I'm sceptical. Old road gearsets are not intended for big cassettes. I don't think running a 14-28 would be a good choice as you are outside the takeup capacity by 2 links and above the max sprocket by 4. Get things which are suitable for what you want to instead of forcing granny gears onto a racing derailleur.
So before sending the Peugeot off for respraying/rechroming I wanted to trial a few freewheel combinations, thus when the frame returns in about 20 weeks time I'd be set to start building without any messing about. By sheer chance I found a 7 speed 14-28 freewheel in the bargain bucket at Halfords for the princely sum of £1! I mounted it on the Campag hub, tweaked the Simplex (Gipiemme) derailleurs a tad and hey presto, it works. Now it ain't perfect but it certainly is completely functional. The reason for the "granny gears" is that I'm hoping to ride the Eroica Gaiole next October and I'm pretty sure I would struggle to haul my aging ass up those Tuscan climbs in 42x24 but with 42x28 there's a chance!
I started spannering at bikes 6 years before this frame was built and the one thing I've learned in that time is that occasionally you have to ditch the manual, have a tinker and see how it turns out!
Belgian Flag S-Works Tarmac viewtopic.php?f=10&t=144553
"Sometimes you don't need a plan. You just need big balls." Tom Boonen