Before I stick my foot in my mouth with non-applicable advice, what kind of bike and what kind of components are you talking about? Is a picture possible?

Okay, now I can stick my foot in my mouth.

Some stem shifters have smaller holes to accommodate the smaller bosses on the shifter clamp, but usually the stem and downtube shifters are the same casting, with extra parts to make 'em work on the downtube (of course, you need either bosses down there or a downtube clamp...) The shifters will usually work with the proper mix of parts, but a set of clamp-on campy shifters for the downtube usually goes pretty cheap on eBay (like $15...suntour power shifters are better and cheaper yet). After the shifters are installed on the bike all you need to do is shorten the derailleur cables.