By the time the tail lifts there are significant aerodynmic loads on the fins. I guess that is enough to keep it running straight.
I used the Great Planes "Large 60" size dural gear for mine. That may sound large, but is just about perfect IMO. The 14 inch wheel spread matches the stock gear. Line the wheels up under the leading edge. The gear tucks in neatly just to the rear of the former at the front of the radio compartment under the wing (F2?). Just add an extra 1/8 sheet of ply from scrap and some blind nuts, voila... like it was meant to be. I used some 1/2 inch tri stock to beef up the inside of the fuse around the mounting plate.
Here is the tail dragger picture that influenced my decision to convert it:
http://webpages.charter.net/rcfu/Kit...SigLT40TD.html