.60 overbore on a 1968 340

There are really no pro's except having straight walls. Con's are weaker cylinder walls that flex more under a load. Flexing cylinders mean less ring seal which in turn means a power loss. Definetly have it sonic checked because you never know what you can get away with until you know how thick the walls are. I've seen a few blocks that were ok to bore .100 over and others that were only safe at .030 over.

The old controversy of an engine bored out excessively will run hot will be a controversy forever. Some guys claim it can't happen while others (me included) say it can because the thinner the cylinder walls are the easier the heat disapates into the cooling system. I might be all wet on my thinking (no pun intendeed) but that's my thoughts on it. Take it for what you will, I'm not a professional engine builder but have built a # of engines and saw on at least 1 occasion a buddy that had his 327 Chevy block bored .060 over and he had a hard time keeping it cool afterward when before he didn't. And before anyone says it's possibly because the clearances were tighter after the rebuild I can say for sure they were just barely tighter. It was already .030 over and didn't need bored again but he was greedy for the extra 5 cubic inches.