The center section ribs are most likely in an angled back postions to meet up when the dihedral is in place so this would need to be filled in. For started head to the hobby store and get a nice piece of ply similar to the brace and make a rectangular shaped piece the width of the original brace so it will fit in between the spars

If you sand down the brace made for dihedral that came with the arf you will lose strength in the final modified piece as it will have progressively less width as you move out to the ends. (see pic).

As for the center section, you could fill it with a balsa wedge. Since your new to ARFs, the easiest would be to find a piece of trailing edge stock that would fit best and place additional balsa shims for a good fit. Rough cut the piece as best possible then sand to meet the contour of the wing.

As a side note.. . I built the TT 40 kit some time ago and with all that dihedral you needed to use the rudder for a nice co-ordinated "grooved" turn. Removing most of it I feel will make a better flying plane. I would leave just a tad though as it it being a trainer with a high wing and very fat airfoil, I feel it will fly better than a zero dihedral wing.