I have had a .60 sized engine on a .40 big stick. An old Fox Eagle I which weighs just a bit more than the K&B .60, needed just one ounce of tail weight. Actually the .40 sized Big Stick is almost as large as the old .60 Ugly Stick. About 2 or 3" less span. The K&B .60 weigh's just a little more than a .50 engine so I would chose that engine.