Why use the axle vent instead of the diff. cover in the higher location? Besides the differentials, are the earlier axles different the the later models years like the Boss used?
When you accelerate hard, the lube in the diff will transfer to the rear of the case. Especially when the oil is hot, it will force its way out of the vent. Capping the diff vent and placing an expansion system off the axle vent will let the thermal expansion of the oil take place, but not force it out to atmosphere.
Bingo....