I have the exact same setup and my engine is mounted upside down. Engine runs like a swiss time piece. Idle is great and the power is super.

I used the 90 header then the stoc header to muffler to try to keep everything as clean as possible. You'll notice in the second pic, I used a Dubro fueler and place a remote glow adapter on the small portion of the fire wall.

I would recommend putting your servos for the elevator and rudder in the rear of the plane for balance as I still had to add 1oz weight to the tail because of the weight of the 91.

Just my 2 cents
Mike