garrettrl,
First, welcome to RCU and the terrific world of RC flying!

You probably should have left that original tank with the "rubber sheet". Chances are the tank was wrapped in foam to isolate it from vibration. As Rob mentioned, you don't want the fuel to "foam" because it will cause the engine to run lean, and quit, and make it hard to tune.... It does not have to be blocked with balsa.

The plane should be balanced around 25% of MAC (Mean Aerodynamic Chord), but don't panic. Find the thickest part of the wing, where it meets the fuselage. That should be the approximate balance point.

To be sure, measure the total wing chord where it meets the fuse, and then measure back 25% of that total from the leading edge. It should come out close to the thickest part of the wing.

Now put a mark on the bottom of the wing, on each side, at that point (against the fuse). Pick the plane up and try to balance it with one fingertip on each of the marks. It should balance level, or slightly nose down. If it balances tail down, move your battery pack forward, or add weight to the nose until it balances properly.

"A nose heavy plane may fly poorly, but a tail heavy plane flys once."

Now, you HAVE found a local club and will be finding someone to teach you how to fly this, right?? Don't try it alone. Or if you do, bring a large trash bag for the pieces.

Good luck and keep us posted.
Dennis-