Lay the parts out on the plane while balancing before deciding where everything goes.
Remember that the fulcrum will be at the recommended center of gravity and the farther away from that point the less weight you will need to balance. You may find you will only need 1/2 oz or less in the very end of the tail and can mount the servos in the original position. In other words just because you have a heavy engine in the front does not mean you will need alot of weight in the rear.
In fact the heavy engine may even be helping you. I have found that most models that I have had to add any weight to for balancing have been tail heavy to start with.