Yeah, I have done the T50.
 
Depending on which T50 box you use (there are 3 types from memory) with each one having the gear stick in a different position.
 
You will need to move the gear stick hole.
 
You will also need to make the tunnel slightly bigger. I split my tunnel down the middle and welding in a strip about 25mm wide. That was enough for me.
 
You may also need to massage out where the clutch fork runs as it can fowl on the tunnel as well.
 
Nothing to difficult, but it not a direct swap by any measure.

You can fit it in without tunnel mods I spose.
 
That just means it all sits lower, and with the 1000 front cross member in the way, the sump would foul i would expect.
 
The T50 is a good box for the 1k, espcially since they can be had for around $500 or less, are stronger than the datsun 5 speed and more common. 
 
The datsun 5 speed only needs minimal mods by comparision. some room for the clutch fork and an adaptorto make the gearbox mounts work.
 
The reverse switch cant be used on the datsun 5 speed as it pokes into the tunnel.