Even if you don't get into the water jackets, the cylinder walls will be too thin and will actually flex and not allow a good ring seal. It will not perform to it's full potential and can eventually break. I don't know what the minimum wall thickness is, but you won't have it with a 318 block bored out that much. Sonic checking is the only way to make sure you have a "safe" cylinder wall thickness and any thicker ares should favor the "thrust" side of the cylinders..
If you don't break into the water jackets with the overbore, you may do so with the cylinder sleeves. Some people don't have a problem sleeving a block to restore a damaged bore. But most people will agree that sleeving a block to get a bore bigger than it was designed for is not a good idea. It will reduce the over strength of the block. I believe 318 main caps are also a bit smaller and weaker than on a 340. Same size, but less metal is used.
I'm sure Mopar blocks are very rare in Europe. But you would be better off trying to find a 360 block which might be easier to find than a 340 and has a bore closer to a 340. It has a cast crank, but the main journals are bigger and stronger than a cast 318/340 crank. A 360 can make and handle a lot of HP, as long as you don't get carried away with the HP goals and rpm's.
