i think most of the problem with putting a 60 on a 40 sized airframe when it comes to a trainer isnt structural its balance. the supertiger 61 is 8 ounces heavier than the 45 for instance. i think you will find it will work just fine you just have to tinker with the balance a little. could also move servos into the tail etc for more weight to balance with. i frankly have only had one plane i used lead to balance with. i do everything i can to get balance with stuff that needs to be on the plane. trainers i think are overbuilt to begin with.

ps the OS 61 SF i think was only 19 ounces so you might not have balance problems after all with the battery balanced aft of the cg.