Chris,
Neat winter building project. As a suggestion, you might want to calculate the horizontal tail (stab) area with respect to the original wing area and determine its percentage. Then consider enlarging the stab area to the same percentage as the elongated wing. The vertical (fin) area should also be increased but you can probably achieve this with a simple dorsal addition to the existing fin. The aerodyam experts can probably chime in on these mods also. You'll enjoy flying this modded Spectra once winter abates here in the northeast.
Rgds,
Art ARRO