A lot actually. Quite a few sxs double shotgun frames are not suitable for conversion to a double rifle. They simply were not built strong enough to handle rifle cartridges that operate at higher pressures. Double rifle frames are usually heavier and have more metal in critical places. Sometimes that extra metal reinforcement is visible in the form of bolsters. Some conversions might seem OK for awhile, but quickly go off face.
Here's a link to a book that covers the subject and all considerations pretty well. It names suitable frames, and explains why. It's been awhile since I looked at my copy, but I seem to recall that the author provided sources for barrel blanks, ribs, sights, etc.
https://www.amazon.com/Building-Double-Rifles-Shotgun-Actions/dp/0971480206
