I would have guessed that the XC would work pretty well for friction shifting. Do you still have the 6 speed 13-30 New Winner freewheel? I'm not sure if that was standard width or Ultra (narrow) but guessing it was standard. Pretty sure for indexing you could use any long cage SunTour Accushift derailleur (such as XCD 6000 or XCE 4050) with Cyclone down tube shifters. SunTour also made 6 speed Accushift bar end shifters, but you will likely have to pay more for them than downtube shifters. I have built my '85 ST500 with 7 speed indexed SunTour, using an XC Pro derailleur, GPX shifters, and SL cassette hub. Pics below. This could also be done with a SunTour freewheel. I have lots of SunTour stuff on the shelf and can test run a couple of setups if needed.
Another option would be to convert to a 7 speed Shimano setup. Pretty sure your frame has 126 mm rear spacing, which should allow for a swap to a 7 speed freewheel with no modifications. Hard to find any ratios other than 14-28 or 14-34, but those two are easy to find new. Use most any long cage Shimano 7/8/9 speed era derailleur and 7 speed DT, bar end, or STI shifters. You will need a MTB derailleur if you want the 34 cog. I have gone the 7 speed Shimano route with my '85 ST400. Pretty sure I have a 14-28 freewheel, XT derailleur, and SL-BS50 bar end shifters.