If you're converting from stem to down tube shifters as I did - and I highly recommend it - Nishiki (and probably other bike makers) made it easy for you. There is a heart-shaped protrusion on the underside of the down tube. It's a stop for the down tube shifter clamp. Place the new shifter clamp so that it rests just above this heart-shaped stop. Works great.

On the advice of others here, I chose and purchased a NOS Suntour Power Shifter assembly off Ebay. It also works great, much better than the Shimano assemblies of the day. Good luck, have fun.