Mostly I agree with syd.
But, growing up, I did a lot of those conversions. The main thing with those bikes is the deraileur has to have a certain type of mounting plate, since the frame lacks a standard mounting hanger. That limits your commonly available equipment unless you can locate an adapting plate. They can be had from the right source, or you can stick with a normal SIS derailleur of that level. I would just as soon do that option. The more important thing with an indexing conversion is to have the right cockpit controls. Are you looking at clicking thumb-shifts or rapid-fire?