The thread alignment tool should be close to the correct bore size, but the problem is Ruger bbls are famous for having off-center bores. I don't have mine handy right now, but IIRC the OD of the bbl is larger than 1/2" at the 16" point so it would have to be turned down to work with a die.
Unless you get real lucky and have a perfectly centered bore the only correct way to get a 10/22 threaded is to use a lathe. The bbl is cut off and faced, then placed between centers. The OD is reduced to the major diameter of the threads needed then the threads are single point cut with a 60 degree threading bit.