Whoo Hoo another Supertigre fan! I just put a GS-40 in my Avistar. Are you sure about the weights? The GS-40 is around 19 oz with muffler. The LA 40 is a VERY light engine for its size. As stated before, try and move stuff around first, adding weight is the last resort. Your receiver battery is the heaviest thing in the airplane, move it where you need it but for God's sakes make sure it's SECURE wherever you put it! Does you no good from a balance standpoint if it's sliding around! I wound up moving battery back about 6" or so, it's balanced perfectly, no weight required.
And if you haven't run it yet, you'll be amazed at the difference in pull between the LA and the S-T. I havene't had a chance to fly mine yet.... hopefully this weekend.

Best of Luck!

Andy