Nice tourer! ...for a tall guy, at least. That frame's on the large size for us average-sized folks.

If the bike fits and you want to explore shifting options besides downtube shifters, going to modern integrated brake/shift levers ("brifters") might be an expensive proposition. If I'm not mistaken, your bike is a 1990 Cannondale ST400, which came equipped with SunTour Accushift shifters, freewheel, and derailleurs. Those are long discontinued and aren't directly compatible with any indexed shifting components available nowadays. Going to brifters or any modern indexed shifting system would require changing out not just the shifters, but the derailleurs and freewheel/cassette as well.

You can switch to bar-end shifters ("barcons") for a much smaller price tag, though. If I remember correctly, SunTour Accushift indexed barcons are plentiful on eBay. Or you could go with just about any bar-end shifters, so long as they can operate in friction mode, and most can. There are some less-common possibilities as well, like Gevenalle shifters.