I've used old Suntour derailers with Shimano indexed levers a couple of times, but there was a catch in that these pre-index Suntour (or shimano or even Campy) derailers feature a " Shimano Dura-Ace 74xx-type" actuation ratio.
So one should either use 74xx index levers with same # of cogs and clicks,
OR
...use any non-Dura-Ace Shimano indexed 7 or 8s lever with a regular-spaced 6s cogset to achieve proper indexing motion with any pre-index-era rear derailer.
OR
...use a Suntour (non-3xxx-series) 7s Accushift lever together with your pre-index derailer and any good index-compatible 7s cogset.

I recommend sticking with the wider standard 5/6-speed cog spacing when using older derailers that weren't made for indexing (though one might get decent shifting using 7s spacing if the cogs are ramped and in their proper sequence).

Never try to make an Allvit derailer index, because it's actuation ratio is unfortunately non-linear.

But even a Gran-Turismo rear derailer will index standard 6s spacing using 6s Dura-Ace 74xx levers (or using Shimano non-Dura-Ace 7/8s index levers), but shifting will be unresponsive unless one drills a new spring anchor hole or two to alter the A/B pivots spring tension balance (so as to bring the top pulley closer to the cogset).
The setup below is not being helped by the sheer length of the old, race-worn cabling up to the non-D-A Shimano 7s bar-end levers (DT levers would shift much better).