Couple ways to do this:
* Cannondale drilled a hole in each side of the downtube, then used a piece of threaded rod and threaded DT mounts (one side was two pieces, so it could be snugged up against the tubing). If you can find one of those mount sets (they are no longer made), you're good. Done properly, it will not compromise the integrity of the tube.
* Depending on how large the DT is, buy an old set of clamp-on DT shifters and use newer DT shifters in place of the friction ones. If the tube is too large for the clamp, you may be able to have a machine shop make a new and larger one for you.

Not a big fan of welding these mounts to the frame, but you could epoxy them if both surfaces were prepared properly.