>>1283273A 70s bike would be friction shifting anyway, unless it's some crappy early version like Positron (unlikely), or it got updated along the way.
>>1283226I'm not really sure what exactly the problem is from the description. If it was upgraded to index shifters and the indexing is off, it probably needs the cable tension adjusted, which you can tweak with the barrel adjuster on the rear derailleur. Or the derailleur hanger could be bent out of alignment (a fairly easy fix on a steel frame). If it's friction, you might just be over/undershifting a bit and you need to fine tune the derailleur position when that happens.
