Bugger me! - I just posted that same link on another thread!
You can get C-Record era 6-speed indexing shifters. I happen to have one. I have never tried to use a Nuovo Record rear derailleur with indexing as all the literature states that derailleurs made before indexing was standard had too much slop to work.
When Campag introduced indexing with their "Synchro" shifters in the mid 80s, their intention was to make it backwardly compatible, so that people would not need to go out and buy new cables/derailleurs/cranks, etc. It was a disaster, and they gave up on it. They ended up redesigning all their drivetrain components with higher tolerences than the old stuff, and indexing finally worked with these newer compnents.
Anyway, back to the matter at hand... If a Nuovo Record rear derailleur is going to work with any indexing system it is going to be 6 or 7-speed. The gaps between the sprockets is widest for these old screw-on systems at a whopping 5.5 mm. I would suggest getting hold of later model Synchro 2 shifters, which are the ones with the larger barrel for the rear derailleur. Also, you will need to find a 6-speed notched insert, which matches your freewheel.
Avoid these Synchro ones:
Get these Synchro 2 ones:
is a lis of all the inserts. Somewhere I have a guide which matches the colours with the various freewheel manufacturers. I'll see if I can find it...