Our doorbell transformer is powered via an original 1950s-era fabric jacketed 2-conductor NM (internal conductors are plastic-jacketed). Photo below shows that NM leaving electrical box; doorbell transformer is sitting out in the open on a wooden shelf on the other side of the visible wood enclosure.
As part of replacing the doorbell transformer to power a Ring doorbell, I'd like to mount the new transformer to a proper metal junction box, which will be secured to a stud in the unfinished garage wall. There is about 3-4 feet of this old but in good condition NM running in the wall to the transformer. I can think of two main choices for connecting the old NM:
Just run it to the new metal junction box and use some sort of box-entry clamp to secure it at a knockout hole. Would I use a modern clamp designed for NM-B, or some special clamp?
Protect it for that short run by running it into something like a liquidtite or flex conduit section (maybe 3/4" trade size?) for the run to the junction box. How would I secure it at the transition to conduit? And would I remove the outer fabric jacket and just run the internal plastic insulated conductors in the conduit to the junction box?
diagram of transformer, side view with 120V connections up:
