I would stay with the 37s. There is such thing as too big. Club I wheel with, a lot of us are on 38 and largers and 1 guy with 42s and another on 52s with ZF axles. If I would do 39.5s on a Track/Kick I think I would go about 66" WMS - WMS axles and stretch the front 3 and 6 in the rear.
Its got a mean stance now, I think 39.5s would hinder more than help.