Fitted H150 diff out of datsun sunny B310 to my B20.  Diff fits in 1000 housing but had to make a diff gasget 2mil thick          Ratio 3.889 which is the tallest diff that fits   Had to us tailshaft out of the B310 because biger diff flang on diff .  You can buy gasget paper 2ml thick from a bearing shop cost me $30 dollars for 1mt so you can make a lot

              
 
 
My ute has a rocker stocker motor so it has lost sum go off the mark but to drive it. It is much better
I have a 1200GX 5 speed I just recode it has no overdrive it is 1to 1 in top gear giving a closer gear ratios
Hopefully a lower first gear .This box fits strait in with no modifying the gearbox mount so will be perfect

There is also the 3.7 H150 from some Sunny wagons
 
Question: would the combo of an overdrive box AND a 3.7 diff be overkill? ( used behind a stock A15)

A15 4 spd and 3.9 works fine in the wagon