Not know much about SBC's, could this be different block with harder cylinders? Seems to me I have read somewhere the material is different which makes the cylinders harder for longer life, this could explain the rings not seating if the walls are not honed correctly.
I'm betting it won't seal up, after 500+ miles, it should have by now. I have never used synthetic oil or blends for break-in, its been said its too slick for the rings to seat. But you can't just throw in any oil due to the flat tappet cam, so ask the ring manufacturer what they recommend.
Good luck,
GM used several different alloy blends for cast iron blocks, some with higher nickel content, some with higher tin content. The mid-seventies block I used to build a 383 has 10% nickel for a longer lasting bore and 10% tin for better control of expansion and contraction. The break-in procedure and type of oil is the same for all of them.
