If they are not IB5 and just late BC box's then there is nothing to be gained really.. The gearbox's are identical apart from the larger IB5 bearings, extra case webbing and self centering neutral.... The late BC box's had the larger bearings too.
But from experience the IB5's take much more power with out breaking, case flex is a big factor with snapping input and main shafts IMO and old fatigued brittle gear sets will often sheer teeth.
I've broken 4 BC box's running around 190bhp and not broken any IB5's running circa 270bhp.