On a 42" barrel I would go with three tenons minimum, and sometimes do 4. If you do three, place one roughly 3" from the muzzle, 8-9" from the breech, and split the difference, which should avoid the narrowest part of the barrel. What size is the bore and how heavy is the breech and waist? Dovetails don't need to be any deeper than about .04" for a rifle barrel, and I usually run them shallower yet. Soft soldering is fine if you're not confident in your dovetail abilities.
-Eric