I have contemplated this on a 4" N frame, and yes, 3.25" was about the minimum I figured you could cut to not mess up the locking lug underneath. This was for a 28 - if your ejector rod and lug are the same then that would be your length.

I have been looking for a cheap/trashed 27, 28, 25, or '37, or '17 for this project myself for a few months. Since I'm not a machinist, it will get a bit expensive on the barrel and sight relocation. But as soon as I find an el-cheapo donor gun, then I am going to go at it. I can round the grips myself, but cutting a barrel, re-crowning it, re-sighting it etc., is something that really needs to be professionally done. Guys here have shown me their home work on refinishing and with some careful, slow work, you can do your own nice rust blue job that will be credible. Not easy - lots of metal prep, careful application, building an impromptu humidor for "sweating", etc. - but entirely doable. So with a cheap gun and just the barrel work, I could have the desirable RB N frame snubbie in .45acp in blue that I want to go with my 629 3", and not be out a huge fortune.

If you do this right, it could be very cool, in my opinion.