There are a lot fo areas to look at as far using oil worn valve guides, valve seals, intake gaskets.
If your cylinders were plate honed and a good cylinder hone was used there should not be any problem with ring seal, We see up to .004 in distortion when the torque plate is bolted on.
We have seen engines on the dyno that guys did not have them plate honed and its unbelievable the differance in blowby at full load and wide open throttle. And these engines really seem to be down on torque and HP with that much blowby.
We build a lot of circle track engines and are breathere are always a dry and see othere engines with rags wraped around the breather from blowby issues and even at the end of the season there is still blowby issues which tell you the rings never seated and with .004 cylinder wall distortion how can they seal.
Stock engines are not as criticle but when your using performance cams that build more cylinder pressure, and smaller CC combustions chambers it magnafies the problem.
We use Clevite bearing gaurd on our cylinder walls with no issues so far and where we plate hone our cylinders with a good hone there is really not much break in for the rings.
Do a leak down test that sould tell you if you are have a ring seal probblem as we see around 1.5 to 2 % leak downs on our engines after a season of running if that helps.
And going to a .040 over bore there is more distortion compared to a smaller bore.
Here are some good links to look over.