Brownells force 44 is tin with 5% silver and has a lower strength than silver brazing. I have done them both ways, solder, with force 44, and threading; I always feel better with threads. I don't think that threading the monoblock weakens it enough to matter; all the ones I have threaded have not noticed.
Make sure you don't confuse hoop (radial) strength, with axial, linear forces. With our modern chrome moly barrels, radial strength is of little concern. And with the relatively low pressures in double rifle calibers, linear forces are easy to deal with as well. All of which is why it is possible, safely, to make double rifles on shotgun frames.