Explain history and the problem better.

But are you trying to suggest that a short time after fitting the gearbox, the thrust bearings inside the engine have failed ?

Are both sides of the thrust damaged ?

Or do you mean the main surface of the centre main ?

Presumably you checked input shaft length of the box before bolting up ? Presumably you didnt pull the bellhousing onto the engine using the bolts forcing the two together when perhaps they really didnt want to go together ?

What are all other bearings like ? What is oil pressure like ? etc etc