The UR-5s work pretty well. I have used them on and off over the years when I could not find Autolite 23s. Some rave about them...there is certainly nothing wrong with them.
Use Autolite 24s unless you are running pretty good boost.
I think you have to do that..just swap them out...some people think the U groove gives a better spark.....I like them or the Aujtolites better than ACs....but it may be more imagination than fact