Howdy
I own a pair of Merwins. Never before tried interchanging their barrels. So I just went downstairs and tried. No soap. The wrong barrel absolutely will not fit on the wrong frame, at least not with my pair.
Now, I must hasten to add, one was originally chambered for 44-40 and the other for 44 Russian. Both now sport custom made cylinders chambered for 44 Russian. Before I read the above post I did not realize the 44 Russian cylinder was shorter, and sure enough it is, by about 1/8". So that probably accounts for why my barrels do not interchange.
I wish you the best of luck and hope your new barrel fits. However, I would not be surprised if some fitting was required.
Please report back on your results.