Al,
My recommendation is still measure the wall thickness. If you can't measure, then by all means use basic guidelines like "no more than 0.060 inch overbore". When you have access to technology and measurements at low cost, use them. You can run closer to the limits while controlling your risks. The mold cores of the cylinder blocks do shift some during casting, so you could have a block with cylinder walls over a range of thickness, either good or bad.
One important question is what is the minimum wall thickness recommended for your engine and application? I have seen figures from 0.100 inches (European engines non-thrust wall) to 0.250" for American V8 special heavy duty racing blocks. One Jeep site listed 0.180" on the thrust side.
For what its worth, I have only heard from several places of boring the 250 to 4.000", 0.125 overbore, to install Chevrolet 327 pistons. Those walls must be mighty thin with the 327 pistons.
