I am 5'7" in height and have a 31.2 in seam and run a 50cm frame. I have longish arms. I use a 110 with road or bullhorns and a 120mm stem MTB bars. With a a track frame I should be using 120/135mm, but I like this a bit shorter and higher on the FG. Without hoods I like the shorter cockpit.

One the road bike its 110mm stem with bars that have a 90mm reach.

I would start around 100mm if you don't want to be too stretched. What's the TT length on that frame?