... I am having trouble balancing the plane and adding too much weight because of the heavy tail. ...
Where are you trying to put the balance point? With that ginormous tail, I'm penciling out a balance point about 80% of the wing chord back from the leading edge (note that this is eyeballing the stab area at 1/2 the wing area -- if it's less, the starting CG needs to go forward!). If you're trying to put it 25% back, or "on the spar", it's not back nearly far enough.
Having said that -- usually free flight competition planes put the balance point way further back than would be sane for RC (I suspect that thing had it right on the trailing edge). So if there's a balance point on the plans that's intended for free-flight, you definitely want to go farther forward than that, and that definitely requires shoving all the gear as far forward as you can, and then maybe extending the nose. For the actual balance point I'd go with my guess above (or I'd build a chuck glider, see below), and I'd be ready to adjust the balance point forward or back.
Here's my source:
http://www.zenithair.com/kit-data/ht-90-4.html Note that I only ever use this as a starting point, and always fine-tune the CG after test flying. Moreover, if I don't trust my numbers I grab some foam or scrap balsa and make a chuck glider to verify the balance point. Usually I find out that I should have trusted my numbers.
