Rebarrel a Ruger No. 3

Nope, barrel and extractor will do it.

Good luck. I’m elk hunting with my No.3 30-40 this year. Great rifle.

I would check the headspace afterwards. Like with most any swap there will be minor differences in the chamber.

Lots of answers to your question on how much work goes into the swap. If you have a prefitted or take-off barrel, the swap is straight forward, if it indexes properly. No guarantees there. I have never used a take-off on a Ruger single shot, always fitted a blank which entails extra work cutting the extractor clearance.
Fortunately, all of the 1's and 3's I have rebarreled have not been extremely tight or LockTited. You have to grip the barrel in front of the lower receiver extension which is not ideal.