Open up to 4"

I have a 1968 318 Block 3.9100 bore but I want to open up to 4.030. I read in a magazine that the 340 bore and the 318 star with the same casting only are diferent machinery. I can't sonie test the block because I'm in SouthAmerica and I can't find a place to do it. The way I see the cylinder walls, they look like a 0.400 thick
Thanks

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How far can I safely bore my block?

http://www.shadydellspeedshop.com/faq.htm

thank you good info