SB to BB swap

I can honestly say that I have never swapped a K frame just to facilitate an engine change.
K frames of all flavors (within their respective model years) have the same dimensions as far as clearances go- the only basic difference being in the engine mounting "pedestals" themselves, nothing that a little creative engineering with basic fab skills can't overcome. Unless it's a restoration or you're a purist, but then you wouldn't be swapping engines anyway.