The general story about sizing a track frame with a threadless stem is to go a centimeter or so smaller than your road frame. However, this is a theory drafted by road riders. You have to remember that if you want a properly fitted track frame, you are always going to be in the drops, which in effect means you may need a slightly larger frame than on the road, all depending on your flexibility. With a threaded headset and quill stem, you have a higher stack height than for threadless, offset by the option of using a downward sloping track stem. The net plus-or-minus difference for the threaded headset less any downward stem slope depends entirely on the stem. Plus, don't forget that track bars tend to be significantly deeper and have significantly greater reach compared to most road bars. In short, there are probably more things causing you to want to go to a slightly larger frame.

Plus, if you look at keirin riders in Japan, they tend to fit their frames in the sizing styles of the 70's, i.e., with relatively little seat post showing. Again, you get this with a slightly larger frame.

As for top tube length, again you are typically using handlebars with greater reach. And you may want more setback on your saddle for greater comfort and power (this is especially helpful for comfort on a road fixie). However, you probably want to use narrower bars for better maneuverability. To open your diaphragm properly, you need to offset the narrower chest position by raising the rib cage slightly on your torso -- and you accomplish this with a slightly longer stem. All told, your top tube may be fairly close to your road top tube, with saddle setback and longer reach bars extending the effective distance.

All this sound vague? Well, track fitting is just different from road. It isn't a tweaking. Your road position is a good starting point, but don't expect to stay there. Track position tends to push your position a little more to extremes, plus you're always on the drops and some of the hardware (like quill track stems) have different dimensions. Sorry I can't be more definitive. Fitting someone is done quickly on the track or even on a fixie ride, but it is hard to do any way short of that.

EDIT: All this just addresses how the frame geometry fits your own personal body geometry. Next there's handling. If you're on the road, you really want to keep your trail at a reasonably high level or the bike will be quite unstable to ride. But you're not likely to find this kind of geometry on a keirin frame. Keirin geometry (with trail down from perhaps 56-58 mm to 30-36 mm) actually makes them stabler than a road geometry on banked tracks -- a road bike would want to drift away from the pole line, while these frames keep a line very nicely. To adjust for different distributions of body weight on a rider, and to adjust for different bankings and turn radiuses, one can tweak stem length a bit -- a longer stem creates more of a lever to help you force the front end through radical steering maneuvers and creates a slight bit of understeering that means you can steer to and stay on the pole line. When the track gets really steep, a short stem can simply be too twitchy and won't help you control the bike properly. Thus on really short tracks with lots of maneuvering (think European six-day tracks, the ADT track in Los Angeles, Alpenrose in Portland, Montreal, etc.) you want a very short trail combined with a somewhat longer stem. The longer stem may not necessitate a shorter top tube to even things out, because on a steep track you want to be stretched out a tad more so you aren't diving down over your handlebars -- the geometry and handling issues sometimes start looking like those on mountain bikes, oddly enough -- you want to scoot your butt back and be reaching out a bit farther forward if you're having to maneuver. What all this means is simply that once again, if you're picking your frame to ride on the track, you may need to tweak it a bit just to go from one track to another. And to ride it on the road, you may not find the trail you need, which is as important as the fit to your body. Again, apologies for being vague, but there are a lot of variables working here and keirin frames aren't typically built to road dimensions unless someone actually had it built just as a road trainer.