If it is going fully into gear and there is no drive then it will be the gearbox I am afraid.
If it is simply baulking when trying to engage 6th gear than as above, it will require an adjustment to the shifter plate.
However, I recall a few weeks ago, a forum member had a selection issue after fitting a new clutch and it transpired that the shifter plate had been refitted the wrong way round so have a look at that.
The stop part of the plate should be on the driver's side as per in the image.
if this is the case then all you will have to do is flip the plate and adjust as required.