Originally Posted by ShannonM

If you don't need indexing, which is unlikely to work with that derailleur no matter what you do, and you don't want to keep the bike all original, which you clearly don't, keep the Shimano levers on the bike that they fit, because that's likely to be the only frame you'll ever own that they fit, and upgrade your derailleur to Suntour. The Suntour derailleurs from that era shifted better than anything else. Upper-end Suntour derailleurs from the early late 1970s and 1980s are, to this day, the best friction-shifting derailleurs ever made by anyone. The Shimano levers will shift the best that they're capable of with a Suntour RD, and the really great Suntour levers didn't come out until later, and they don't fit your bike without adapters anyway, so you're not losing anything.

--Shannon