I have the 71 230 block and it had a couple thin cylinders. #1 and #8 i think. .120 on the thrust of #1 before it got bored. Best Machine told me to fill it, so it got a tall fill with Hard Block and aluminum main caps added. Been running this as a 511 for 4 seasons now. Mostly street use, but has been down the track a couple dozen times now. I believe its under 700hp at this point and no issues with it yet. I do keep a spare 77 400 block just in case.