Shimano's 6speed line was typically designated with the 105X (105 level), 6400 (600/Ultegra). There's also the RX100 out there that would work too. Shimano 6speed freewheels are still readily avaliable, as are the shifters (loosescrews.com was just throwing a sale on them, also often found on ebay).

dgregory is right though, the SIS stuff did appear later - late 86/87 sounds about right from what research I've done in the past. The only problem I've run into with running indexed shifters on older frames is the lack of shift bosses on the downtube to mount them to. Many frames didn't have any, others which did were not the right style for later Shimano levers. There are clamps out there with the correct boss style attached. One that comes to mind is the clamp that comes with the Shimano 105 Golden Arrow shifter (also sold on loosescrews and often seen on ebay). The Shimano 600 shift levers of simular age to the G.A. 105's are very abundant on ebay, but haven't seen with my own eyes that they have the same style clamp though.

Hope this helps a bit.