I posted about my experience with repro synchro cones
here, although this relates more to the manufacturing quality of 3rd/4th rings. Planning to give the rebuilt gearbox another run in a few weeks, hopefully with better results than last time.
Can you identify which type of synchro cones went into the gearbox? The better quality type have fine machining marks running across the outside surface whereas the others have more of a cast/forged look.
Also, is it possible that 2nd gear shift fork is not quite sitting in the right place in neutral so there is always slight pressure to hold the synchro onto its gear? Or maybe too much endfloat is allowing the mainshaft to move rearwards... guessing a bit there. You'd see a lot of brass in the oil if it had worn down from overloading.
Andy.