I had a guy have a s-w 66 4in. sleeved with a 1/4 in or maybe 1/2 in extension on the front, SS of course, looked like crap, all drilled out and a liner installed, full length.
Worked for a while , till he , or his brother loaded up 357mag. and the forcing cone split. was O.K. with 38 sp.
I think if a gunsmith does that, he should stop the sleeve / liner just a head of the cone, so you have full sized steel on end of the barrel.
the steel left after the hole is drilled for the liner is real thin, and so is the liner, so no strength at the cone.
The fellow that did it is retired and took him about 2 yrs to get it done.
I don't have the shop now, but that is one thing to think about.
Who ever does it has to have a 12-6 licence and the paper work has to be sent in with lots of pictures.
If you want I can measure the colt barrel again, the smith was 36 tpi if I remember, I just compared the colt and way different, and yes, 357 model and python are the same frame, so should be the same thread. I don't have a 357 model barrel out and not going to take mine out. but do have the python barrel out .
The 357 is a simple barrel, would not be that hard to fit one, You could even stamp the roll mark/ model in with some time and right punch.