I'm not sure of how your engine is set up, but some engines have copper washers between the block and head where the oil galleys are. Forget to replace them or have one crushed and that could be the problem.
Are there O rings around the cylinder sleeve? If so, that's called a wet sleeve and can let coolant leak into the oil, but not usually the other way around. How about a copper gasket under the top lip of the sleeve? While very highly unlikely, there could be a casting flaw between an oil galley and coolant passage. Whatever you do, don't run it with antifreeze in the oil because you'll glue the engine up completely. Just as my brother about that.
