Agreed.
When you sleeve a shotgun with rifle barrels you have to lay ribs and regulate it anyway....
Sooo.... re-regulating a Sabatti .400 or .450, or laying ribs on a Baikal .45-70, will probably always be easier.

If you already have a good donor shotgun, then OK, but to go looking for a cheap one to sleeve to .45-70 when the Baikals are sitting right there for under a grand, well.....!

Having said that, donor shotguns need to be robust. Pretty-much what TennHillBilly said, but look for a pigeon or waterfowl gun, NOT a game gun. IMHO length of action bar, length of front lump, width of the lumps, and width/height of standing breech will dictate potential far more than whether it has a cross-bolt, or side-clips, or not.