6-bolt fishplates can't be used with bullhead rail because they would force the timbers too far apart to properly support the joint. Unlike flat-bottom, bullhead fishplates must fit between the chairs and keys, not over the top of the baseplates. That's why 2-bolt fishplates were sometimes used -- to get the timbers closer to the joint. For normal bullhead track, 4-bolt fishplates with adjacent timbers at 24" or 25" centres are near universal.
Martin.