If you have to get clamp on indexed shifters I'd just stop there because it's not really worth the hassle. Sounds like you'd need a shimano freewheel (7 speed might not fit your axle length), shimano indexed shifters, some kind of clamp that will let you mount indexed shifters on them, a rear derailleur, new cables, a new rear cable housing. OR you could just use what's on there and not be out much if anything performance wise.