Jeff,
The (I)O-340 was a certified engine at one time, not just a modified O-320. I would suspect that it is as reliable as any of the Lycomings. Smoothness has a lot to do with balance (and engine mount) and I'm sure any of the engine builders can balance it to match an O-320 or O-360.
Based on your signature, I would guess you are building a -7. If that is the case, why not go with an O-360? The price is about the same, you get a few more HP, and it is more common. (That common thing is probably why you don't year much about the 340.)
