I believe that the best value for money 5 speed gearbox for a datsun is still the toyota T50 box.

They fit into the existing tunnel with only minor mods (bolting locations)
are as expensive as chasing down a datsun 5 speed (~$1000 including the belhousing)
will need a custom propshaft (toyota output to datsun diff housing)
and are considered stronger that the datsun box.

Datsun box
has a 2 different shift patterns (std and dogleg)
has several ratio gear sets (some overdrive 5th others 1:1 5th)
can be hard to track down
pricey at around $700-900 for a reasonable 2nd hand unit.
still needs aaptors made to fit the bolting points
fits into std tunnell, but the reverse switch will need to be modified/removed.
can use a std datsun propshaft.