Sharps-nut,
Good question. Generally speaking, no reputable gunsmith would rechamber a 30-30 to a 308 Win. But.....there are other .308 cartridges that are safe for Contenders. It has more to do with chamber pressure
than bore diameter. The 300 Savage is considered safe, I believe. The 309 JDJ is safe as is the 308 Bellm and several others. Most 'smiths who rechamber Contenders will know what is safe and what isn't. You won't go wrong if you use SSK, Dan Van Horn, Virgin Valley, Bullberry or Mike Bellm. Most of the folks who frequent here know what is safe as well. If you have a question about a specific chambering, just ask.
However, if you only chose chamberings available from TC Arms, you'd really limit your choices. Go to the VVCG and Bullberry websites and look at all the available chamberings. ......lots of safe choices
Rick