I obsess about this type of thing.
The T-700 is a serious bike, and it was outfitted with some of the best equipment available at the time.
If you've got the original 34T sprocket back there, that limits what derailleurs you can use. Even stuff with a great reputation for smoothness and reliability are usually built to handle 28 or 30T.
The unit that was on there was Shimano's top of the line, high capacity unit. I look at that whole period of 1974-1994 as "period correct."
I have one bike with a 34T cog- I have used Suntour XC Pro and XC Comp derailleurs on there. They're really from 1990-1993, but they look at home on mid-80s touring bikes- I have an XC Comp on my 1985 Trek 620 and 1986 Trek 400 Elance and have used (and may go back to) an XC Pro on my 1984 Voyageur SP. Personally, these look so cool AND they function great as an indexed derailleur, AND when used with 6sp Accushift- they function equally as well. They have a great range, fantastic chain wrap capability and a strong spring that makes for really positive shifting.
XC Comp on the Trek 620:
XC Pro on the Voyageur SP:
Right now I have a 1st gen Cyclone GT on the Voyageur SP. It's nice, it's fine, it shifts nicely- and it's outrageously beautiful. It's really super light and... it's really beautiful. I haven't had it a long time and really don't have a strong opinion- I've wanted to use one for a long time, and it looks right at home on this bike.
Speaking of the VSP (which I do incessantly), it came with a Suntour LeTech. I don't have any miles on it- because I wanted to use the XC Pro- and, truth be told- I don't like the computer font. But it is an innovative, if overkill, 3 pivot system. At the time of the 1984 VSP- the LeTech was the top of the line touring derailleur from Suntour.
In 1986, I think the top of the line Suntour touring derailleur was the XC- I don't have one of those and haven't ridden one. My other Class A touring bike is a 1985 Trek 720. It came with a Sachs/Huret DuoPar. My first experience with the DuoPar was really bad. For the life of me, I could not understand why anyone would want such a horrible device on their bike. When I got the 720, I had plans to replace the DuoPar with an XC Pro; but based on the experience of John D Thompson, I opted to give the DuoPar Titanium a chance. I'm really amazed at how smoothly this shifts. I've used some of the top of the line derailleurs, not just touring or MTB units- this 1975 design, coupled with the maligned Helicomatic system is THE smoothest shifting bike I have ridden. I mean, seriously wondering if the shift happened because there is no 'click' or 'chunk' or anything- it just slides into gear. I'm hesitant to recommend it just because of my (and some other) horror stories about it- however it really should be mentioned.
You really can't talk about rear derailleurs without mentioning Shimano's XT units- from the Deerhead model that came on yours to the M730, 732 and M735- they're (IMO) the
standard of "'business' class" SIS rear derailleurs. The M735 looks almost as cool as the XC Pro/XC Comp, shifts fantastic in friction, and shifts indexed from 6-8 speed. There's no losing, using XT.
Just because of the range, good looks and functionality in friction and Accushift (lets me use Command Shifters)- I would recommend the XC Pro or XC Comp derailleurs. They're Class A equipment all the way.