So i've just finished putting my bus back together, and the gearstick feels 'stuck'. Everything went together smoothly, and it is a newly built box which shifted fine when out of the car (could move the shift shaft easily)...

What are the obvious things i should stat checking first?

Thank you

I assume that you have checked the obvious things like the front mounting is the correct one, the front bush is OK, the ball hasn't popped out of the shift rod at the front, the shift rod is dragging on something. from there let the coupling go and see if it is then free if not it has to be in the shift housing.

the gate plate could be wrong way in or misaligned and when you tightened the 13mm bolts it may have locked all up.
try loosening and shifting, things may pop into place without full removal.