One of the blocks I recently picked up was a 400 - 4 bolt that is
already 30 over. There is surface rust and a ring lip in the cylinders so it will need to be machined.

The question is can a 400 be machined to 60 over without making the walls too thin. One of the books I have (How to Build Big-Inch Chevy Small Blocks) says it's a bad idea.

Anyone with some experience in this area?