Xj400 4v7 sixth gear noise and whine

Good day all!

I recently bought a xj400 4v7 that i will be making a little cafe/brat build on. It was properly cheap but still in decent condition. Cylinder base gasket leaking a bit of oil, trip meter isnt working (mileage is though) and the "big issue" is what the post is about.

The sixth gear has some noise when not under loaf noises and whines a bit under load. At regular speeds you can't hear it but riding slower on sixth gear makes it noticeable.

Gonna rip it apart soon and have a look, but i was thinking i should ask around. Is it common with sixth gear noises? Is there a specific part that tends to fail? Should i just order a gearbox right now because it will be f-ed? It seems to be very similar to the xj550 seca (4v8). And the gearbox internals look identical based on images. Maybe it even is the same?

Cheers!
Oh, if someone knows where to get an exploded view with article numbers for both the 400 and 550 that would be of great use.