Bicycle Mechanics - Index Shifting/ Rear Derailleur Question

I'm trying to convert my 1988 Bertoni Nuovoitalia from downtube friction shifting to Shimano Index shifting with SIS thumbshifters. At this age it's only got a 6-speed freewheel (14-28), so I got a 6-speed thumbshifter. The original rear derailleur is a Shimano 600 (RD-6207), and I've tried to get it dialed in. It shifts from lower gears to higher gears just fine, but back up to the lower gears is very poor. I thought maybe I needed to get an SIS rear derailleur, so I tried out just a cheaper Tourney SIS (TY-30), and that does just the opposite, higher gears to lower fairly smoothly, but lower to higher is very poor. Question: Will a better quality 6/7 speed SIS RDER like maybe an Ultegra do the trick? Any other suggestions? Appreciate any help. Thanks!!:)

Ummmmm........chotto muzukashii desu ne.....

Hey George. Bikkuri ****a:eek: ! Nihongo dekiru ne!
I'm trying to figure out if it's better to match a road RDER to the road freewheel, or if it's maybe better to match a shorter cage MTB RDER to the Tourney Thumbshifter. Do you think the throw on the MTB SIS thumbshifter needs to match the throw of an MTB SIS RDER?

Well first off if it was originally friction chances are that setup was not index compatible. You not only need to have shifters and derrailleurs you need the freewheel and chain as well. Also if you need a higher end 6 speed thumbie (XT)let me know.

I agree with Hunter. Your freewheel needs to be index compatible. Have you thought of just upgrading your drivetrain to 9 speed.......meaning getting a new rear casette hub also? Seems like you have a very nice frame worth the upgrade. Kono houga kantan jia nai kana...?

Jia mata,
Joji