Of the 172's you see on floats most are the hawk xp models, probably a good reason for that but if your gonna do it anyway then good luck to you. My advice would be keep the weight down and consider the aircraft a 2 seater you
might be OK. Also try a climb prop and a STOL kit, also there is an stc for the 140 to bring it up to 160 horse's. If it's the really old pa28 (with the trim handle on the roof) you may not have the factory corrosion proofing of the later models.
See the Cherokee Pilots Association web site maybe there's some info there.
http://www.piperowner.com/