Remove the lead. measure where the wing should be placed for it to balance correctly. It will be aft of where it is now. Build new one with wing in this position. Otherwise figure out how to get 4-6 oz out of the tail. It is a simple ratio of the distances from the cg point. Typically the nose is about 1/4 as long as the tail. An oz of weight on the tail will take 4 oz of weight on the nose to counter. Try to take some weight out of the design as 13 lbs for a 75" wing plane is still a bit portly.