[ted]

How much you can bore a 390 block will depend largely upon what year block you're dealing with. Typically, the newer the block, the thinner the cylinder walls. Some of the earlier blocks could actually get away with 0.080 over and make a standard 406 or 428 block but I've seen the later blocks actually hit water by way of casting flaws or pin holes in the cylinders at 0.030 over.

I would recommend getting the block sonic checked for both cylinder wall integrity and core shift as then you'd know for sure. Knowing which direction the core shift resides is invaluable when trying to bore one of these engines to the max as the better machine shops can offset bore the cylinder into the thicker portion of the shift and subsequently recenter the bore so the cylinder walls are more equal in all directions.