down tube shifters are a world better than stem mounted. the shorter cable run and the almost complete lack of cable housings make the shifting sweet. Your symmetrics were offered in both braze on mounting and clamp mounting...and I'm pretty sure that they made a band for the oversized tubing. The band was specific, though. I'm sounding like an eBay advert today; search for suntour symmetric and see what you find. Local bike shops once had these different sizes in stock, maybe you'll get lucky!

Oh, and bar ends are pretty nice; look for the suntour power ratchet shifters. Cheap, plentiful (at least on eBay) and dependable.