The original Flattop .44 Mag frame is the same size as the Super Blackhawks. The Flattop .357s are the ones built on the smaller mid sized frame.
The SBH cylinder of either vintage may or may not fit and function in the Flattop. There are many tolerances that come to play. Total length of the cylinder from the ratchet to the boss at the front, the length of the boss, the length of the cylinder body and the ratchet fingers as well.
If the replacement cylinder is too long sometimes they can be fitted easily, sometimes not. If it is too short you'll have to machine it for a new boss at the front to set the end shake.
Then you have to consider the barrel cylinder gap.
Lots of things to fitting a cylinder.
Or, it just might drop right in and fit perfectly. I've had it happen but it's not something to expect.