Wing engineering?

Ok, a question or 2 about the wings. I'm building a wooden aircraft
that could go either fixed tri-gear or retract. I choose fixed seeing
how this is my first plane to build and I wanted to keep it simple. The
fuel was supposed to go behind the firewall but I choose to build tanks
in each wing where the wheels where to go.
What will this do to the loading of the aircraft? Does this effect
the useful load and how? Does it raise my useful load or lower it or
niether? Since the wings create lift, and the fuel was in the fuselage
and now in the wing should it screw up the center of gravity? Anybody,
Anybody, Anybody????