Minimum tail shaft length
 

Im doing a conversion on my car and im worried the tail shaft that has been made up is too short. Dose anyone know what the maximum length of slip yoke showing out of the back of the box is?
The reason i ask is because, my gearbox is leaking at the rear seal. I was thinking maybe the slip yoke is not going far enough into the box to seal properly?
The slip yoke is new and the car has not been started yet. The leak is only slow, making a pool of about half a coffee mug after 3 months.
If its not the length could it just be the rear seal in the box?