I run a Saito 56 on mine and it was about the limit on being nose heavy. The battery is under the servos and the reciever is next to them.

Horizon talks about putting a Saito 72 in the 40.

Your OS 70 should be fine if you move the battery back in the tail. Place it on the fuselage until the plane balances and that's where you put it.

I really like the planes characteristics. It's my favorite plane right now.

Enjoy,

Jim