Using receiver blocks (instead of barrel blocks) is the RIGHT way to install a barrel, BUT regardless of how you install it, there are a few things that you need to do to ensure that the barrel is properly aligned.
First, you need to have some moly-based lubricant for the barrel nut, and you'll want to apply it both to the threads and to the collar at the bottom that rubs against the collar on the barrel. This helps reduce the torque applied to the barrel.
Second, make sure that the pin is in good shape, and replace it if not. You can also use small squares of aluminum cut from soda cans to shim the pin.
Third, if possible, have someone support the barrel while you tighten the barrel nut. You should tighten and loosen the nut twice before doing a final torque on the third time. This helps seat the threads properly and reduces the chances of the barrel being loose.
Finally, you can use a guage (if you have a flattop, you'll need a detachable carry handle). Brownells sells them, but it's really a piece of drill rod with a pointed end that you clamp into the channel on the carry handle. If the point is pointing straight at the front sight, you're good. If not, then you know how far the alignment is off, and can shim or adjust the barrel as needed.
-Troy