Depends on the reason for a build. If you like to hunt deer size critters up close then a 20 bore rifle is ideal. The Fox action is plenty strong enough for a bore rifle. I like projects like this and you end up with a safe and easy to use conversion. The most readily available rifled tubes come from hastings and have a very fast rate of twist only suitable for sabots. If it suits your needs it looks like a fun build.
Steve