im building a profile (scratchbuilt) that is very similer to the tower fun-51. it has about the same airfoil only the wing has been extended 2 bays making it about 52 inches. (compared to 44)
the fuse has been extended about 4 inches and the rudder/elevator servos will now be in the tail. (its about 3 feet 3 inches)
heres a pic of the fuse.....
do you think i should sheet it? im thinking of throwing a magnum 46 or 52 for the engine. the plane will be for 3d (if it even flies). the fuse is made out of 3/8in balsa while the origional th fun 51 used 1/4 balsa for the fuse and it was sheeted

also should i fiberglass the center section of the wing since its much bigger?

I vote for sheet.

Even very thing balsa skin will add a lot of strenght and rigidity.

Recently, Bob Hunt wrote several articles in AMA magazine about this in the CL section.
He recommends lite-fiberglassing over the skin, as well as the center of the wing.

If you can keep your wing loading about the same use sheeting.  Weigh some pieces of sheeting and see what you get. You have expanded the model about 30% but wieght might not increase proportionately but one of the critical parameters for 3D is wing loading.