The plane apparently balances out ideally with a stock build using an OS61FX and probably a plastic engine mount. Your Saito 100 is about an ounce heavier than the 61FX, add the extra weight of the aluminum mount, and you're a little nose heavy. You should be able to offset the extra weight with placement of the batteries.

I would fully assemble the plane before covering the fuse and installing the tail servos and control rods, and figure out where the battery should be placed. If the battery has to be placed too far back in the fuse, mount the rudder/elevator servo's in the rear. Then re-balance again before covering, and figure out where the battery should go.

That's the step I'm at myself.

Chuck