Some blocks have been drilled between cylinders. You drill at an angle back to the water jacket. If you look a lot of the circle track heads had water passages drilled between cylinders. Most head gaskets have two small holes between cylinders this is where the hole starts and is angled about 45 degree towards the water jacket.

Simply because you are asking I would say do not do this. This mod is for someone who is fully aware of the problems they may cause and what bore they can get away with after doing this.

Leon